fix warning: invalid conversion specifier 'H' [-Wformat-invalid-specifier]
https://rt.cpan.org/Ticket/Display.html?id=145110

Index: lib/Devel/MAT/Dumper.xs
--- lib/Devel/MAT/Dumper.xs.orig
+++ lib/Devel/MAT/Dumper.xs
@@ -1160,7 +1160,7 @@ static void dumpfh(FILE *fh)
     HE *he;
 
     if(hv_iterinit(legacy_helper_per_package)) {
-      warn("Legacy per-package helpers in %HELPER_PER_PACKAGE are no longer followed\n");
+      warn("Legacy per-package helpers in %%HELPER_PER_PACKAGE are no longer followed\n");
       while((he = hv_iternext(legacy_helper_per_package))) {
         STRLEN len;
         warn("  %s\n", HePV(he, len));  // package names won't contain embedded NULs
@@ -1168,7 +1168,7 @@ static void dumpfh(FILE *fh)
     }
 
     if(hv_iterinit(legacy_helper_per_magic)) {
-      warn("Legacy per-magic helpers in %HELPER_PER_MAGIC are no longer followed\n");
+      warn("Legacy per-magic helpers in %%HELPER_PER_MAGIC are no longer followed\n");
       while((he = hv_iternext(legacy_helper_per_magic))) {
         STRLEN len;
         warn("  %s\n", HePV(he, len)); // magic addresses are decimal integers
