selftest/intel_pstate/aperf: Use LDLIBS instead of LDFLAGS

Build of aperf fails as below:
```
gcc  -Wall -D_GNU_SOURCE   -lm  aperf.c  -o /tools/testing/selftests/intel_pstate/aperf
/tmp/ccKf3GF6.o: In function `main':
aperf.c:(.text+0x278): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status
```

The faulure occurs because -lm was defined as LDFLAGS and implicit rule
of make places LDFLAGS before source file.  This commit fixes the
problem by using LDLIBS instead of LDFLAGS.
Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/intel_pstate/Makefile

 CFLAGS := $(CFLAGS) -Wall -D_GNU_SOURCE
-LDFLAGS := $(LDFLAGS) -lm
+LDLIBS := $(LDLIBS) -lm
 
 TEST_GEN_FILES := msr aperf