powertop: Fix build for !uclibc

* EXTRA_LDFLAGS isn't defined for !uclibc and configure fails
  when it reads it unexpanded, see config.log snippet:

  configure:4177: checking whether the C compiler works
  configure:4199: i586-oe-linux-gcc  -m32 -march=i586 --sysroot=/OE/sysroots/qemux86  -O2 -pipe -g -feliminate-unused-debug-types  -Wl,-O1 -Wl,--hash-style=gnu -Wl,--as-needed ${EXTRA_LDFLAGS} conftest.c  >&5
  i586-oe-linux-gcc: error: ${EXTRA_LDFLAGS}: No such file or directory
  configure:4203: $? = 1
  configure:4241: result: no

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>