cpu/x86/mp_init.c: Avoid static analysis error for unused value

Within procedure save_bsp_msrs, the structure pointer "msr_entry" is updated
every time procedure save_msr() is called. However, after the last call of
save_msr(), "msr_entry" is not used, thus causing a static analysis error.
Add a "(void)msr_entry;" at the end to avoid the static analysis error.

BUG=b:112253891
TEST=Build and boot grunt.

Change-Id: If0fb336fbf49eec3da255fadbe38b3a38768d0cf
Signed-off-by: Richard Spiegel <richard.spiegel@silverbackltd.com>
Reviewed-on: https://review.coreboot.org/27956
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/cpu/x86/mp_init.c

@@ -297,6 +297,9 @@ static int save_bsp_msrs(char *start, int size)
 
 	fixed_mtrrs_hide_amd_rwdram();
 
+	/* Tell static analysis we know value is left unused. */
+	(void)msr_entry;
+
 	return msr_count;
 }