kselftest/arm64: mte: Makefile: Fix clang compilation

kselftest/arm64: mte: Makefile: Fix clang compilation

When clang finds a header file on the command line, it wants to
precompile that, which would end up in a separate output file.
Specifying -o on that same command line collides with that effort, so
the compiler complains:

clang: error: cannot specify -o when generating multiple output files

Since we are not really after a precompiled header, just drop the header
file from the command line, by removing it from the list of source
files in the Makefile.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Mark Brown <broone@kernel.org>
Link: https://lore.kernel.org/r/20210319165334.29213-10-andre.przywara@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>