selftests/intel_pstate: Update makefile to match new style

Recent changes from Bamvor (88baa78d) have standardized the
variable names like TEST_GEN_FILES and removed the need for make targets
all and clean.

These changes bring the intel_pstate test inline with those changes.

Cc: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: NStafford Horne <shorne@gmail.com>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/intel_pstate/Makefile

-CC := $(CROSS_COMPILE)gcc
 CFLAGS := $(CFLAGS) -Wall -D_GNU_SOURCE
 LDFLAGS := $(LDFLAGS) -lm
 
-TARGETS := msr aperf
+TEST_GEN_FILES := msr aperf
 
-TEST_PROGS := $(TARGETS) run.sh
+TEST_PROGS := run.sh
 
-.PHONY: all clean
-all: $(TARGETS)
+include ../lib.mk
 
-$(TARGETS): $(HEADERS)
-
-clean:
-	rm -f $(TARGETS)
+$(TEST_GEN_FILES): $(HEADERS)