Fix hexdigit conversion in the pcre parser

The pcre parser in the dfa backend is not correctly converting escaped
hex string like 
  \0x0d

This is the minimal patch to fix, and we should investigate just using
the C/C++ conversion routines here.

I also I nominated for the 2.7 series.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Seth Arnold <seth.arnold@gmail.com>

parser/libapparmor_re/parse.y

@@ -169,7 +169,7 @@ int hexdigit(char c)
 	else if (c >= 'A' && c <= 'F')
 		return 10 + c - 'A';
 	else if (c >= 'a' && c <= 'f')
-		return 10 + c - 'A';
+		return 10 + c - 'a';
 	else
 		return -1;
 }