LibHTML: Make it possible to build LibHTML on the host.

- "make" builds the normal Serenity libhtml.a
- "make -f Makefile.host" builds a test program for the host machine.

LibCore/CFile.cpp

@@ -1,4 +1,5 @@
 #include <LibCore/CFile.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>