selftests/arm64: Define top_srcdir for the fp tests

Some of the rules in lib.mk use a top_srcdir variable to figure out where
the top of the kselftest tree is, provide it.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220427181954.357975-3-broonie@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/fp/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -I../../../../../usr/include/
+# A proper top_srcdir is needed by KSFT(lib.mk)
+top_srcdir = $(realpath ../../../../../)
+
+CFLAGS += -I$(top_srcdir)/usr/include/
+
 TEST_GEN_PROGS := sve-ptrace sve-probe-vls vec-syscfg za-fork za-ptrace
 TEST_GEN_PROGS_EXTENDED := fp-pidbench fpsimd-test \
 	rdvl-sme rdvl-sve \