LibCore: Explicitly declare `environ` in Process.cpp to unbreak macOS

Userland/Libraries/LibCore/Process.cpp

@@ -8,12 +8,13 @@
 #include <LibCore/Process.h>
 #include <errno.h>
 #include <spawn.h>
-#include <unistd.h>
 
 #ifdef __serenity__
 #    include <serenity.h>
 #endif
 
+extern char** environ;
+
 namespace Core {
 
 pid_t Process::spawn(StringView path)