perf: Fix undefined PyVarObject_HEAD_INIT in python 2.5

PyVarObject_HEAD_INIT is undefined in python 2.5, resulting
in a build crash:

	util/python.c:81: attention : déclaration implicite de la fonction « «PyVarObject_HEAD_INIT» »
	util/python.c:82: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:117: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:146: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:177: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:290: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:359: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:532: erreur: request for member «tp_name» in something not a structure or union
	util/python.c:761: erreur: request for member «tp_name» in something not a structure or union
	error: command 'gcc' failed with exit status 1
	make: *** [python/perf.so] Erreur 1

We can fix that by defining PyVarObject_HEAD_INIT as a wrapper on
PyObject_HEAD_INIT, thanks to a trick found on biopython:
https://github.com/biopython/biopython/commit/d4eaf57946c7b4c32eca8d18821edf32f83e300d



Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>