os: fix mising prototype / include on WIN32 builds

[90/383] Compiling C object os/liblibxlibc.a.p/strcasestr.c.obj
359../os/strcasestr.c:45:1: warning: no previous prototype for ‘xstrcasestr’ [-Wmissing-prototypes]
360   45 | xstrcasestr(const char *s, const char *find)
361      | ^~~~~~~~~~~

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1295>

os/strcasestr.c

@@ -37,6 +37,8 @@
 #include <ctype.h>
 #include <string.h>
 
+#include "os.h"
+
 /*
  * Find the first occurrence of find in s, ignore case.
  */