vapigen.m4: use $PKG_CONFIG_SYSROOT_DIR
vapigen.m4: use $PKG_CONFIG_SYSROOT_DIR

This is necessary in cross-compiling environments, where directories
returned by pkg-config should be prefixed with sysroot location.

frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala