proxy-libintl: Put .a in -dev package and allow empty main package

This helps in staging the library when it is asked for
via dependencies otherwise its missing in staging sysroot
even though its built its stashed into -staticdev package

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>