eolian_cxx: add missing libgen.h include for basename() to fix build on OS X

On Linux this went through and has gone unnoticed. basename() needs libgen.h
though and eolian_cxx was not including it.

src/bin/eolian_cxx/eolian_cxx.cc

@@ -5,6 +5,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <getopt.h>
+#include <libgen.h>
 
 #include <string>
 #include <algorithm>