perf build: Fix perf build issues with fixdep

perf build: Fix perf build issues with fixdep

Commit a808a2b35f66 ("tools build: Fix fixdep dependencies") broke the
perf build ("make -C tools/perf") by introducing two inadvertent
conflicts:

  1) tools/build/Makefile includes tools/build/Makefile.include, which
     defines a phony 'fixdep' target.  This conflicts with the $(FIXDEP)
     file target in tools/build/Makefile when OUTPUT is empty, causing
     make to report duplicate recipes for the same target.

  2) The FIXDEP variable in tools/build/Makefile conflicts with the
     previously existing one in tools/perf/Makefile.perf.

Remove the unnecessary include of tools/build/Makefile.include from
tools/build/Makefile, and rename the FIXDEP variable in
tools/perf/Makefile.perf to FIXDEP_BUILT.

Fixes: a808a2b35f66 ("tools build: Fix fixdep dependencies")
Reported-by: Thorsten Leemhuis <linux@leemhuis.info>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://patch.msgid.link/8881bc3321bd9fa58802e4f36286eefe3667806b.1760992391.git.jpoimboe@kernel.org