allocators: fix meson build after physmem move from -bad

gst-libs/gst/allocators/meson.build

 gst_allocators_headers = [
   'allocators.h',
   'gstfdmemory.h',
+  'gstphysmemory.h',
   'gstdmabuf.h',
 ]
 install_headers(gst_allocators_headers, subdir : 'gstreamer-1.0/gst/allocators/')
 
-gst_allocators_sources = [ 'gstdmabuf.c', 'gstfdmemory.c' ]
+gst_allocators_sources = [ 'gstdmabuf.c', 'gstfdmemory.c', 'gstphysmemory.c']
 gstallocators = library('gstallocators-@0@'.format(api_version),
   gst_allocators_sources,
   c_args : gst_plugins_base_args,