Replace obscure <corecrt.h> with standard <stddef.h> in win32 for size_t

See https://en.cppreference.com/w/c/types/size_t
Hence, this may later become a generic solution rather than a platform specific one...

platforms/win32/vm/sqPlatformSpecific.h

@@ -24,7 +24,7 @@
 
 #if defined(__MINGW32__) || defined(__MINGW64__)
 // vcruntime.h defines size_t for MSVC builds, but not for mingw builds.
-# include <corecrt.h> // for size_t
+# include <stddef.h> // for size_t
 #endif
 
 /* File positions in the FilePlugin & other plugins */