Fix vfork test strcmp buildbot failure (#84224)

The buildbot seems to complain about `strcmp` function not available in
the vfork patch (#81564):

https://lab.llvm.org/buildbot/#/builders/68/builds/70093/steps/6/logs/stdio

Unfortunately, I can't reproduce the failure on my linux machine so this
is a guessing fix. If anyone has a way to reproduce and very this fix,
please feel free to merge this change.

Co-authored-by: jeffreytan81 <[email protected]>

Author: jeffreytan81

lldb/test/API/functionalities/fork/concurrent_vfork/main.cpp

@@ -2,6 +2,7 @@
 #include <cstring>
 #include <iostream>
 #include <mutex>
+#include <string.h>
 #include <sys/wait.h>
 #include <thread>
 #include <unistd.h>