package/at-spi2-core: fix build with meson 0.50.0

package/at-spi2-core: fix build with meson 0.50.0

Add upstream patch [1] to fix build with meson 0.50.0 (as suggested
by Thomas Petazzoni ).

Fixes [2]:

  atspi/meson.build:60:0: ERROR: Subdir keyword must not be an absolute path.

[1] https://github.com/GNOME/at-spi2-core/commit/44a812ea51223d82f21a098a2d45fcc5c329ce7a.patch
[1] http://autobuild.buildroot.net/results/83250522c79b5749fc8236ba2b0e873a12747413

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>