bball: fix multiple definition linking error

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.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>

testcases/bball/tga_reader.h

@@ -30,8 +30,8 @@ typedef struct _TGA_ORDER {
 	int alphaShift;
 } TGA_ORDER;
 
-const TGA_ORDER *TGA_READER_ARGB;
-const TGA_ORDER *TGA_READER_ABGR;
+extern const TGA_ORDER *TGA_READER_ARGB;
+extern const TGA_ORDER *TGA_READER_ABGR;
 
 void *tgaMalloc(size_t size);
 void tgaFree(void *memory);