vala: Fix srcdir != builddir
vala: Fix srcdir != builddir

We need to look for the .metadata file in the source directory,
otherwise we silently generate a binding with a namespace of "Tp", not
"TelepathyGLib".

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=49802
Reviewed-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib