powertop: only link against -lintl when needed

BR2_PACKAGE_GETTEXT=y might be enabled even if BR2_NEEDS_GETTEXT is
not set, and in this case, we should not link against libintl, because
it may not exist, for example with glibc toolchains.

Fixes:

  http://autobuild.buildroot.net/results/270/270018e2c9e3845a4015faa155325eea2cabe3d9/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>