pgrep: Fix format security

do_regcomp() error message didn't use a string literal for the format string.

pgrep.c:538:24: error: format not a string literal and no format arguments
[-Werror=format-security]

Signed-off-by: Craig Small <csmall@dropbear.xyz>

pgrep.c

@@ -534,7 +534,7 @@ static regex_t * do_regcomp (void)
 
         if (re_err) {
             regerror (re_err, preg, errbuf, sizeof(errbuf));
-            xerrx(EXIT_USAGE, errbuf);
+            xerrx(EXIT_USAGE, _("regex error: %s"), errbuf);
         }
     }
     return preg;