gitignore: ignore GNU GLOBAL tag files
gitignore: ignore GNU GLOBAL tag files

GNU GLOBAL (https://www.gnu.org/software/global/) is source code
tagging system.  It creates 4 tag files (GTAGS, GRTAGS, GSYMS and
GPATH) for the symbol cross-reference.  Ignore them.

Signed-off-by: Masahiro Yamada <378b411a8a63ecc7605ec4272234862d7781c6ae@socionext.com>

achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,sbranden/arm-trusted-firmware