LibC: Add vfork() as a simple wrapper around fork()

I don't know if we should implement optimized vfork(). I'm adding it
here since SDL2 uses it.

Userland/Libraries/LibC/unistd.cpp

@@ -89,6 +89,11 @@ pid_t fork()
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+pid_t vfork()
+{
+    return fork();
+}
+
 int execv(const char* path, char* const argv[])
 {
     return execve(path, argv, environ);

Userland/Libraries/LibC/unistd.h

@@ -64,6 +64,7 @@ int gettid();
 int donate(int tid);
 int getpagesize();
 pid_t fork();
+pid_t vfork();
 int execv(const char* path, char* const argv[]);
 int execve(const char* filename, char* const argv[], char* const envp[]);
 int execvpe(const char* filename, char* const argv[], char* const envp[]);