externalsrc: move back to classes

The externalsrc class was moved to classes-recipe as part of oe-core
f5c1280, but it can be used in both recipe and global contexts so move
it back to classes/.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

diff --git a/meta/classes-recipe/externalsrc.bbclass b/meta/classes/externalsrc.bbclass
similarity index 100%
rename from meta/classes-recipe/externalsrc.bbclass
rename to meta/classes/externalsrc.bbclass