logrotate: use pkg-config for the popt library

Without using the pkg-config the dependencies are not included for a
static linking so it fails to build.
These failures are fixed by linking the libintl library that is a
dependency of the popt library.

This commit to be useful requires the previous commit 'popt: add to the
"popt.pc" file the libintl library'.

Fixes:
http://autobuild.buildroot.net/results/159bf5730414ca7f73dcdae95090177355193636
http://autobuild.buildroot.net/results/ebe6ab7fc0f2cff98de06b3dc374730da9e9e4f2



Minimal defconfig to reproduce the build failure:
BR2_STATIC_LIBS=y
BR2_TOOLCHAIN_BUILDROOT_LOCALE=y
BR2_PACKAGE_GETTEXT=y
BR2_PACKAGE_LOGROTATE=y

[Thomas: fix commit log, since the library is named 'popt', not
'opt'.]

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>