Escape left brace as new perl suggests

After upgrade to perl-5.22.0, it started complaining about one of our
scripts.  The thing is that even though it works, it wants all curly
brackets escaped properly.  The change is not functional, it merely gets
rid of the following error:

  Unescaped left brace in regex is deprecated, passed through in regex;
  marked by <-- HERE in m/^enum { <-- HERE / at -e line 3.

There is one more error like this that I'm getting, but it is because of
GNU automake bug #21001:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21001

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

src/Makefile.am

@@ -451,7 +451,7 @@ PDWTAGS = \
 	    $(PERL) -0777 -n						\
 		-e 'foreach my $$p (split m!\n*(?:$(r1)|$(r2))\n!) {'	\
 		-e '  if ($$p =~ /^(struct|enum) $(struct_prefix)/ ||'	\
-		-e '      $$p =~ /^enum {/) {'				\
+		-e '      $$p =~ /^enum \{/) {'				\
 		-e '    $$p =~ s!\t*/\*.*?\*/!!sg;'			\
 		-e '    $$p =~ s!\s+\n!\n!sg;'				\
 		-e '    $$p =~ s!\s+$$!!;'				\