Fix missing include on OpenBSD

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 265803
diff --git a/llvm/lib/Support/Unix/Path.inc b/llvm/lib/Support/Unix/Path.inc
index 3cedf64..0ccca59 100644
--- a/llvm/lib/Support/Unix/Path.inc
+++ b/llvm/lib/Support/Unix/Path.inc
@@ -68,6 +68,7 @@
 #else
 #ifdef __OpenBSD__
 #include <sys/param.h>
+#include <sys/mount.h>
 #elif defined(__ANDROID__)
 #include <sys/vfs.h>
 #else