gdbsupport/pathstuff.h: #include <array> explicitly for std::array<>

This fixes build breakage using clang with libc++ on FreeBSD where
std::array<> is not yet declared when used by the path_join variadic
function template.

gdbsupport/pathstuff.h

@@ -26,6 +26,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <array>
 
 /* Path utilities.  */