gdb: Fix building with clang

This resolves the following clang++ error message:

../../gdb/symtab.c:4892:33: error: logical not is only applied to the left hand side of this comparison [-Werror,-Wlogical-not-parentheses]
              if (preg.has_value () && !preg->exec (sym->natural_name (), 0,
                                       ^
../../gdb/symtab.c:4892:33: note: add parentheses after the '!' to evaluate the comparison first
              if (preg.has_value () && !preg->exec (sym->natural_name (), 0,
                                       ^
                                        (
../../gdb/symtab.c:4892:33: note: add parentheses around left hand side expression to silence this warning
              if (preg.has_value () && !preg->exec (sym->natural_name (), 0,
                                       ^
                                       (

Bug: https://sourceware.org/PR31328

Signed-off-by: Jens Remus <jremus@linux.ibm.com>