analyzer: fix release build

I broke the build with --enable-checking=release with
r17-6246-g7647c00b77ee1a; sorry.

Should be fixed by this patch; smoketested with a pair of builds
with checking both enabled and disabled.

gcc/analyzer/ChangeLog:
	* string-scan.cc
	(named_test_bit_vector::scan_for_first_zero_byte): Wrap definition
	with #if CHECKING_P
	* symbolic-bit-contents.cc (class named_test_bit_contents):
	Likewise.
	* symbolic-scalars.cc (class named_test_scalar): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>