package/pkg-cmake: <pkg>_SUBDIR cleanup

No functional change is brought by this modification.

This patch removes redundant <pkg>_SRCDIR declaration
  (already defined in pkg-generic.mk)

Signed-off-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>