Fix building on Linux

It seems that glibc doesn't support strnstr, and strcasestr requires
_GNU_SOURCE to be defined

src/detection/displayserver/linux/wayland.c

@@ -1,5 +1,3 @@
-#define _GNU_SOURCE //required for struct ucred
-
 #include "displayserver_linux.h"
 
 #include <stdlib.h>

src/util/FFstrbuf.h

@@ -159,7 +159,7 @@ static inline FF_C_NODISCARD int ffStrbufIgnCaseComp(const FFstrbuf* strbuf, con
 
 static inline FF_C_NODISCARD bool ffStrbufContainS(const FFstrbuf* strbuf, const char* str)
 {
-    return strnstr(strbuf->chars, str, strbuf->length) != NULL;
+    return strstr(strbuf->chars, str) != NULL;
 }
 
 static inline FF_C_NODISCARD bool ffStrbufContainIgnCaseS(const FFstrbuf* strbuf, const char* str)