SDL_stdinc.h: provide a prototype for strdup for __clang_analyzer__ case

Because strdup is not ANSI but POSIX, and its prototype might be hidden..

Reference issue: https://github.com/libsdl-org/SDL/issues/11219.

(cherry picked from commit 3ebfdb04be8d543321524113951f817e0e2c56c4)

include/SDL_stdinc.h

@@ -749,6 +749,9 @@ size_t wcslcpy(wchar_t *dst, const wchar_t *src, size_t size);
 size_t wcslcat(wchar_t *dst, const wchar_t *src, size_t size);
 #endif
 
+/* strdup is not ANSI but POSIX, and its prototype might be hidden... */
+char *strdup(const char *str);
+
 /* Starting LLVM 16, the analyser errors out if these functions do not have
    their prototype defined (clang-diagnostic-implicit-function-declaration) */
 #include <stdlib.h>