tools: gpio: Use !building_out_of_srctree to determine srctree

make TARGETS=gpio kselftest fails with:

Makefile:23: tools/build/Makefile.include: No such file or directory

When the gpio tool make is invoked from tools Makefile, srctree is cleared and the current logic check for srctree equals to empty string to determine srctree location from CURDIR.

When the build in invoked from selftests/gpio Makefile, the srctree is set to "." and the same logic used for srctree equals to empty is needed to determine srctree.

Check building_out_of_srctree undefined as the condition for both cases to fix "make TARGETS=gpio kselftest" build failure.

---
 tools/gpio/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

index 6ecdd1067826..1178d302757e 100644
@@ -3,7 +3,11 @@ include ../scripts/Makefile.include
 
 bindir ?= /usr/bin
 
-ifeq ($(srctree),)
+# This will work when gpio is built in tools env. where srctree
+# isn't set and when invoked from selftests build, where srctree
+# is set to ".". building_out_of_srctree is undefined for in srctree
+# builds
+ifndef building_out_of_srctree
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))
 srctree := $(patsubst %/,%,$(dir $(srctree)))
 endif

2.20.1