fetchmail: gettext is not mandatory

fetchmail: gettext is not mandatory

Even when locales are enabled, gettext is not mandatory to build
fetchmail, i.e the following defconfig builds fine:

BR2_arm=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_CUSTOM=y
BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD=y
BR2_TOOLCHAIN_EXTERNAL_URL="http://autobuild.buildroot.org/toolchains/tarballs/br-arm-full-2017.02-1096-g54a5333.tar.bz2"
BR2_TOOLCHAIN_EXTERNAL_GCC_4_8=y
BR2_TOOLCHAIN_EXTERNAL_HEADERS_3_10=y
BR2_TOOLCHAIN_EXTERNAL_LOCALE=y
BR2_TOOLCHAIN_EXTERNAL_CXX=y
BR2_INIT_NONE=y
BR2_SYSTEM_BIN_SH_NONE=y
BR2_PACKAGE_FETCHMAIL=y

However, if gettext provides libintl, it gets used. Therefore this
commit moves gettext from a mandatory dependency to an optional
dependency.

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