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.