selftests: ptp: include default header install path

Add the usr/include subdirectory of the top-level tree to the include
path to fix build when cross compiling for ARM.
testptp.c: In function 'main':
testptp.c:289:15: error: 'struct ptp_clock_caps' has no member named 'cross_timestamping'
           caps.cross_timestamping);
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/ptp/Makefile

+CFLAGS += -I../../../../usr/include/
 TEST_PROGS := testptp
 LDLIBS += -lrt
 all: $(TEST_PROGS)