win32: Add missing function setenv

Mingw32 does not provide a declaration and implementation of function
setenv (which is used in sdl.c), so this patch adds both.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

osdep.h

@@ -95,6 +95,8 @@ int qemu_create_pidfile(const char *filename);
 #ifdef _WIN32
 int ffs(int i);
 
+int setenv(const char *name, const char *value, int overwrite);
+
 typedef struct {
     long tv_sec;
     long tv_usec;