Switch to fully-qualified #include

Previously, mosh used extensive -I flags and all of the mosh-local
makes it really hard to tell what the proper dependency graph is, so
instead remove the -I arguments in favvor of $(top_srcdir) and qualify
the paths wherever they are used.

src/util/swrite.cc

@@ -34,7 +34,7 @@
 #include <string.h>
 #include <stdio.h>
 
-#include "swrite.h"
+#include "src/util/swrite.h"
 
 int swrite( int fd, const char *str, ssize_t len )
 {