Don't attempt to include system headers when not required

Some builds don't require system headers: no filesystem and single
threaded.

Author: SparkiDev

wolfssl/wolfcrypt/wc_port.h

@@ -59,7 +59,9 @@
 #endif /* WOLFSSL_LINUXKM */
 
 /* THREADING/MUTEX SECTION */
-#ifdef USE_WINDOWS_API
+#if defined(SINGLE_THREADED) && defined(NO_FILESYSTEM)
+    /* No system headers required for build. */
+#elif defined(USE_WINDOWS_API)
     #if defined(WOLFSSL_PTHREADS)
         #include <pthread.h>
     #endif