tools/net/ynl: suppress jobserver warning in ynltool version detection

tools/net/ynl: suppress jobserver warning in ynltool version detection

When building ynltool with parallel make (-jN), a warning is emitted:

  make[1]: warning: jobserver unavailable: using -j1.
  Add '+' to parent make rule.

The warning trips up local runs of NIPA's ingest_mdir.py, which
correctly fails on make warnings.

This occurs because SRC_VERSION uses $(shell make ...) to make
kernelversion. The $(shell) function inherits make's MAKEFLAGS env var
which specifies "--jobserver-auth=R,W" pointing to file descriptors that
the invoked make sub-shell does not have access to.

Observed with:

$ make --version | head -1
GNU Make 4.3

Instead of suppressing MAKEFLAGS and foregoing all future MAKEFLAGS
(some of which may be desirable, such as variable overrides) or
introducing a new make target, we instead just ignore the warning by
piping stderr to /dev/null. If 'make kernelversion' fails, the ' || echo
"unknown"' phrase will catch the failure.

Before:
	NIPA ingest_mdir.py:

	ynl
	 Full series FAIL   (1)
	   Generated files up to date; build has 1 warnings/errors; no diff in
	   generated;

After:
	NIPA ingest_mdir.py:

	Series level tests:
	 ynl                             OKAY

Validated output:
	$ ./ynltool/ynltool --version
	ynltool 6.19.0-rc4

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://patch.msgid.link/20260112-ynl-make-fix-v1-1-c399e76925ad@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>