compositor: add fallback strchrnul()

Android does not have this function.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/shared/os-compatibility.h b/shared/os-compatibility.h
index b2021f1..c1edcfb 100644
--- a/shared/os-compatibility.h
+++ b/shared/os-compatibility.h
@@ -46,4 +46,9 @@
 int
 os_create_anonymous_file(off_t size);
 
+#ifndef HAVE_STRCHRNUL
+char *
+strchrnul(const char *s, int c);
+#endif
+
 #endif /* OS_COMPATIBILITY_H */