automake: bring back the .PHONY git_sha1.h.tmp rule

automake: bring back the .PHONY git_sha1.h.tmp rule

With earlier commit 3689ef32afd ("automake: rework the git_sha1.h rule,
include in tarball") we/I erroneously removed the PHONY rule and the
temporary file.

The former is used to ensure that the header is regenerated when on each
make invocation, while the latter helps us avoid the unneeded rebuild(s)
when the SHA1 hasn't changed.

Reported-by: Grazvydas Ignotas <notasas@gmail.com>
Tested-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>