Include port_platform in gcp_environment tests.

Both the Windows and Linux tests use platform-specific macros to
determine whether they should execute, but they weren't including the
file where those macros were defined, so they were always running the
else case of that check, which meant they weren't testing anything.

test/core/security/check_gcp_environment_linux_test.cc

@@ -16,6 +16,8 @@
  *
  */
 
+#include <grpc/support/port_platform.h>
+
 #include "src/core/lib/security/credentials/alts/check_gcp_environment.h"
 
 #if GPR_LINUX

test/core/security/check_gcp_environment_windows_test.cc

@@ -16,6 +16,8 @@
  *
  */
 
+#include <grpc/support/port_platform.h>
+
 #include "src/core/lib/security/credentials/alts/check_gcp_environment.h"
 
 #ifdef GPR_WINDOWS