contrib: Fix make_sunver.pl warning

Petr informed me that perl 5.32 bundled with Solaris 11.4 warns about
make_sunver.pl:

Unescaped left brace in regex is passed through in regex; marked by <-- HERE in m/^([ \t]*){ <-- HERE $/ at /vol/gcc/src/hg/master/local/libgomp/../contrib/make_sunver.pl line 216.

I didn't notice since I'm using a common installation of perl 5.12
across Solaris versions that doesn't show that warning.

His patch fixes the issue.  Tested on Solaris 11.3 (perl 5.12) and 11.4
(perl 5.32).

2023-01-20  Petr Sumbera  <petr.sumbera@oracle.com>

	contrib:
	* make_sunver.pl: Escape brace.