include all pj headers in extern "C"

Change-Id: I704857602be0c65751721331707b1a9928f4785b
diff --git a/src/fileutils.cpp b/src/fileutils.cpp
index b6194d2..c8dbdc0 100644
--- a/src/fileutils.cpp
+++ b/src/fileutils.cpp
@@ -71,8 +71,10 @@
 #include <cstddef>
 #include <ciso646>
 
+extern "C" {
 #include <pj/ctype.h>
 #include <pjlib-util/md5.h>
+}
 
 #include <filesystem>