gas: Fix x86 build with GCC 6.4

gas: Fix x86 build with GCC 6.4

Add "()" to silence GCC 6.4:

.../gas/config/tc-i386.c: In function ‘x86_ginsn_lea’:
.../gas/config/tc-i386.c:5738:19: error: logical not is only applied to the left hand side of comparison [-Werror=logical-not-parentheses]
   if (!i.base_reg != (!i.index_reg || i.index_reg->reg_num == RegIZ))
                   ^~
cc1: all warnings being treated as errors

	PR gas/31464
	* config/tc-i386.c (x86_ginsn_lea): Add "()" to silence GCC 6.4.