crt: Silence GCC warnings about missing prototypes

Clang didn't warn about this, which is why it slipped through this
far.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/stdio/snprintf_alias.c

diff --git a/mingw-w64-crt/stdio/snprintf_alias.c b/mingw-w64-crt/stdio/snprintf_alias.c
index 1c34114..b9a892d 100644
--- a/mingw-w64-crt/stdio/snprintf_alias.c
+++ b/mingw-w64-crt/stdio/snprintf_alias.c
@@ -13,6 +13,7 @@
 
 int __cdecl __ms_vsnprintf(char *buffer, size_t n, const char *format, va_list arg);
 
+int __cdecl snprintf(char *buffer, size_t n, const char *format, ...);
 int __cdecl snprintf(char *buffer, size_t n, const char *format, ...)
 {
   int retval;

mingw-w64-crt/stdio/vsnprintf_alias.c

diff --git a/mingw-w64-crt/stdio/vsnprintf_alias.c b/mingw-w64-crt/stdio/vsnprintf_alias.c
index 8c7c9b9..d62a07f 100644
--- a/mingw-w64-crt/stdio/vsnprintf_alias.c
+++ b/mingw-w64-crt/stdio/vsnprintf_alias.c
@@ -13,6 +13,7 @@
 
 int __cdecl __ms_vsnprintf(char *buffer, size_t n, const char *format, va_list arg);
 
+int __cdecl vsnprintf(char *buffer, size_t n, const char *format, va_list arg);
 int __cdecl vsnprintf(char *buffer, size_t n, const char *format, va_list arg)
 {
   return __ms_vsnprintf(buffer, n, format, arg);