don't prototype strdup() for __clang_analyzer__ case in windows builds.

Fixes:  libsdl-org#12948.
(cherry picked from commit 17bba02)

Author: sezero

include/SDL_stdinc.h

@@ -750,8 +750,12 @@ 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
 
+#ifndef _WIN32
 /* strdup is not ANSI but POSIX, and its prototype might be hidden... */
+/* not for windows: might conflict with string.h where strdup may have
+ * dllimport attribute: https://github.com/libsdl-org/SDL/issues/12948 */
 char *strdup(const char *str);
+#endif
 
 /* Starting LLVM 16, the analyser errors out if these functions do not have
    their prototype defined (clang-diagnostic-implicit-function-declaration) */