package/pango: propagate reverse dependencies

It appears some pango reverse dependencies aren't propagated properly.

Fixes:
WARNING: unmet direct dependencies detected for BR2_PACKAGE_PANGO
  Depends on [n]: BR2_USE_WCHAR [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && BR2_USE_MMU [=y] && BR2_INSTALL_LIBSTDCPP [=y] && BR2_TOOLCHAIN_HAS_SYNC_4 [=y] && BR2_TOOLCHAIN_GCC_AT_LEAST_4_9 [=n]
  Selected by [y]:
  - BR2_PACKAGE_GST1_PLUGINS_BAD_PLUGIN_TTML [=y] && BR2_PACKAGE_GSTREAMER1 [=y] && BR2_PACKAGE_GST1_PLUGINS_BAD [=y] && BR2_USE_WCHAR [=y] && BR2_TOOLCHAIN_HAS_THREADS [=y] && BR2_INSTALL_LIBSTDCPP [=y] && BR2_TOOLCHAIN_HAS_SYNC_4 [=y]

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>