bball: fix multiple definition linking error

X11 DISTRO_FEATURE is needed for XWayland distro. Which turns on X11 tests for emc-test-suite.

Build fails with following errors: ld: git/testcases/bball/tga_reader.h:34: multiple definition of TGA_READER_ABGR'; git/testcases/bball/tga_reader.h:34: first defined here ld: git/testcases/bball/tga_reader.h:33: multiple definition of TGA_READER_ARGB'; git/testcases/bball/tga_reader.h:33: first defined here

Add extern to those definitions in header file.

requested review from @jonas.hoeppner

assigned to @mikhail.vanyulin

mentioned in merge request seco-ne/yocto/layers/meta-seconorth-distro!567 (closed)

merged

Integration merge request: seco-ne/yocto/manifest!1169 (merged)

mentioned in merge request seco-ne/yocto/manifest!1169 (merged)