graphics/gegl: Fix build WITH_DEBUG.

ld: error: undefined symbol: backtrace_symbols_fd
>>> referenced by gegl-buffer.c:349 (../gegl/buffer/gegl-buffer.c:349)
>>>               gegl/libgegl-0.4.so.0.427.1.p/buffer_gegl-buffer.c.o:(gegl_buffer_leaks)
>>> referenced by gegl-buffer.c:939 (../gegl/buffer/gegl-buffer.c:939)
>>>               gegl/libgegl-0.4.so.0.427.1.p/buffer_gegl-buffer.c.o:(gegl_bt)

ld: error: undefined symbol: backtrace
>>> referenced by gegl-buffer.c:938 (../gegl/buffer/gegl-buffer.c:938)
>>>               gegl/libgegl-0.4.so.0.427.1.p/buffer_gegl-buffer.c.o:(gegl_bt)
>>> referenced by gegl-buffer.c:922 (../gegl/buffer/gegl-buffer.c:922)
>>>               gegl/libgegl-0.4.so.0.427.1.p/buffer_gegl-buffer.c.o:(gegl_buffer_set_alloc_stack)
cc: error: linker command failed with exit code 1 (use -v to see invocation)

PR:		253788