tools/resolve_btfids: Include libsubcmd headers directly from source tree

Currently each build with resolve_btfids enabled unnecessarily prints
the line 'INSTALL libsubcmd_headers' from libsubcmd.

Use the libcmd headers from source tree instead, without installation.

The same was done for objtool in commit ac999926774a ("objtool: Include
libsubcmd headers directly from source tree"), albeit for a different
reason.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Tested-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://patch.msgid.link/20260702-libsubcmd-spam-v1-1-300ec142a62f@linutronix.de
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>