kselftest/arm64: signal: drop now redundant GNU_SOURCE definition

kselftest/arm64: signal: drop now redundant GNU_SOURCE definition

The definition of GNU_SOURCE was recently centralised in an upper layer
kselftest Makefile, so the definition in the arm64 signal tests Makefile
is no longer needed. To make things worse, since both definitions are
not strictly identical, the compiler warns about it:
<command-line>: warning: "_GNU_SOURCE" redefined
<command-line>: note: this is the location of the previous definition

Drop the definition in the arm64/signal Makefile.

Fixes: cc937dad85ae ("selftests: centralize -D_GNU_SOURCE= to CFLAGS in lib.mk")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240816153251.2833702-2-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>