meson.build: Always look for libsigc++ via pkg-config
meson.build: Always look for libsigc++ via pkg-config

We have Meson build support for libsigc++ before we got into this drive
for Meson support for building glibmm, so let's just always look for
libsigc++ with pkg-config.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm