drm/panthor: Add src path to includes of panthor_gpu.o

The file that defines CREATE_TRACE_POINTS needs to have the src
directory added to its include paths, or else the build may fail, as
define_trace.h won't be able to find the included trace file.

Add it to the Makefile's CFLAGS for panthor_gpu.o.

Fixes: 52ebfd8d2feb ("drm/panthor: Add tracepoint for hardware utilisation changes")
Reported-by: Sasha Levin <sashal@kernel.org>
Closes: https://lore.kernel.org/r/aXLyzd6pMmexwWlY@laps/
Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patch.msgid.link/20260123-panthor-tracepoint-build-fix-v1-1-7d67b7c0ae9b@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>