AArch64/opcodes: Correct an `index' global shadowing error

AArch64/opcodes: Correct an `index' global shadowing error

Fix a commit 4df068de5214 ("Add support for SVE addressing modes") build
regression:

cc1: warnings being treated as errors
.../opcodes/aarch64-dis.c: In function 'aarch64_ext_sve_addr_rr_lsl':
.../opcodes/aarch64-dis.c:1324: error: declaration of 'index' shadows a global declaration
/usr/include/string.h:303: error: shadowed declaration is here
make[3]: *** [aarch64-dis.lo] Error 1

in a way following commit 91d6fa6a035c ("Add -Wshadow to the gcc command
line options used when compiling the binutils.").

	opcodes/
	* aarch64-dis.c (aarch64_ext_sve_addr_rr_lsl): Rename `index'
	local variable to `index_regno'.