meson.build: use relative path

Fixes:
Fatal error: can't create
gdk/gdkmm/libgdkmm-3.0.so.1.1.0.p/_buildarea1_master-wr_build_Userspace_auto-usrmerge_standalone_usrmerge_next_210222_lxbuilds_Har12345_platform_up_intel-x86-64-standard-glibc-std_wrlinux_build_tmp-glibc_work_corei7-64-wrs-linux_gtkmm3_3.24.3-r0_gtkmm-3.24.3_untracked_gdk_gdkmm_timecoord.cc.o: File name too long

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>