[ObjC] Fix potential `-Wstrict-prototypes` warning.

PiperOrigin-RevId: 609718293

objectivec/GPBAny.pbobjc.m

@@ -43,7 +43,7 @@ static GPBFileDescription GPBAnyRoot_FileDescription = {
 // This is to help make sure that the GPBWellKnownTypes.* categories get linked and
 // developers do not have to use the `-ObjC` linker flag. More information
 // here: https://medium.com/ios-os-x-development/categories-in-static-libraries-78e41f8ddb96
-__attribute__((used)) static NSString* any_importCategories () {
+__attribute__((used)) static NSString* any_importCategories(void) {
   return GPBWellKnownTypesErrorDomain;
 }
 

objectivec/GPBDuration.pbobjc.m

@@ -43,7 +43,7 @@ static GPBFileDescription GPBDurationRoot_FileDescription = {
 // This is to help make sure that the GPBWellKnownTypes.* categories get linked and
 // developers do not have to use the `-ObjC` linker flag. More information
 // here: https://medium.com/ios-os-x-development/categories-in-static-libraries-78e41f8ddb96
-__attribute__((used)) static NSString* duration_importCategories () {
+__attribute__((used)) static NSString* duration_importCategories(void) {
   return GPBWellKnownTypesErrorDomain;
 }
 

objectivec/GPBTimestamp.pbobjc.m

@@ -43,7 +43,7 @@ static GPBFileDescription GPBTimestampRoot_FileDescription = {
 // This is to help make sure that the GPBWellKnownTypes.* categories get linked and
 // developers do not have to use the `-ObjC` linker flag. More information
 // here: https://medium.com/ios-os-x-development/categories-in-static-libraries-78e41f8ddb96
-__attribute__((used)) static NSString* timestamp_importCategories () {
+__attribute__((used)) static NSString* timestamp_importCategories(void) {
   return GPBWellKnownTypesErrorDomain;
 }
 

src/google/protobuf/compiler/objectivec/file.cc

@@ -155,7 +155,7 @@ void EmitLinkWKTs(absl::string_view name, io::Printer* p) {
             // This is to help make sure that the GPBWellKnownTypes.* categories get linked and
             // developers do not have to use the `-ObjC` linker flag. More information
             // here: https://medium.com/ios-os-x-development/categories-in-static-libraries-78e41f8ddb96
-            __attribute__((used)) static NSString* $basename$_importCategories () {
+            __attribute__((used)) static NSString* $basename$_importCategories(void) {
               return GPBWellKnownTypesErrorDomain;
             }
           )objc");