libbpf: Add missing per-arch include path

[ Upstream commit 710fbca820c721cdd60fa8c5bbe9deb4c0788aae ]

libbpf does not include the per-arch tools include path, e.g.
tools/arch/riscv/include. Some architectures depend those files to
build properly.

Include tools/arch/$(SUBARCH)/include in the libbpf build.

Fixes: 6d74d178fe ("tools: Add riscv barrier implementation")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240927131355.350918-1-bjorn@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>

tools/lib/bpf/Makefile

@@ -61,7 +61,8 @@ ifndef VERBOSE
 endif
 
 INCLUDES = -I$(or $(OUTPUT),.) \
-	   -I$(srctree)/tools/include -I$(srctree)/tools/include/uapi
+	   -I$(srctree)/tools/include -I$(srctree)/tools/include/uapi \
+	   -I$(srctree)/tools/arch/$(SRCARCH)/include
 
 export prefix libdir src obj