Fix for opencv/opencv#8105 (compiling with mingw32)

fix for opencv/opencv#8105, compilation issue with mingw32 (in
google/googletest#721 a similar issue was solved and the reason was
described as MinGW defines _CRITICAL_SECTION and _RTL_CRITICAL_SECTION
as two separate (equivalent) structs, instead of using typedef)

modules/ts/include/opencv2/ts/ts_gtest.h

@@ -720,10 +720,16 @@
 #  include <io.h>
 # endif
 // In order to avoid having to include <windows.h>, use forward declaration
+# if GTEST_OS_WINDOWS_MINGW
+   // MinGW defined _CRITICAL_SECTION and _RTL_CRITICAL_SECTION as two
+   // separate (equivalent) structs, instead of using typedef
+   typedef struct _CRITICAL_SECTION GTEST_CRITICAL_SECTION;
+# else
    // assuming CRITICAL_SECTION is a typedef of _RTL_CRITICAL_SECTION.
    // This assumption is verified by
    // WindowsTypesTest.CRITICAL_SECTIONIs_RTL_CRITICAL_SECTION.
-struct _RTL_CRITICAL_SECTION;
+   typedef struct _RTL_CRITICAL_SECTION GTEST_CRITICAL_SECTION;
+# endif
 #else
 // This assumes that non-Windows OSes provide unistd.h. For OSes where this
 // is not the case, we need to include headers that provide the functions
@@ -3057,7 +3063,7 @@ class GTEST_API_ Mutex {
   // by the linker.
   MutexType type_;
   long critical_section_init_phase_;  // NOLINT
-  _RTL_CRITICAL_SECTION* critical_section_;
+  GTEST_CRITICAL_SECTION* critical_section_;
 
   GTEST_DISALLOW_COPY_AND_ASSIGN_(Mutex);
 };