LibC: Make dlfcn stubs extern "C" like the outside world expects.

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/631894279b0

LibC/dlfcn.cpp

@@ -1,5 +1,7 @@
-#include "dlfcn.h"
 #include <assert.h>
+#include <dlfcn.h>
+
+extern "C" {
 
 int dlclose(void*)
 {
@@ -20,3 +22,5 @@ void *dlsym(void*, const char*)
 {
     ASSERT_NOT_REACHED();
 }
+
+}

LibC/dlfcn.h

@@ -1,5 +1,9 @@
 #pragma once
 
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
 #define RTLD_LAZY 1
 #define RTLD_NOW 2
 #define RTLD_GLOBAL 3
@@ -9,3 +13,5 @@ int dlclose(void*);
 char *dlerror();
 void *dlopen(const char*, int);
 void *dlsym(void*, const char*);
+
+__END_DECLS