Reinstate include of regex.h for Linux and MinGW build

I'm not sure how boost/regex.h is relevant but it doesn't enable the
build to succeed in on Linux or using Mingw so if this is not an error
and required for msvc build then fix it

libs/pbd/pathexpand.cc

@@ -23,7 +23,7 @@
 #include <cerrno>
 #include <cstdlib>
 
-#include <boost/regex.h>
+#include <regex.h>
 
 #include <glibmm/miscutils.h>