Fixes the following building error, happening when building both intel and broadcom:

Gen Header: libmesa_broadcom_genxml_32 <= v3d_packet_v21_pack.h
FAILED: gen/STATIC_LIBRARIES/libmesa_broadcom_genxml_intermediates/broadcom/cle/v3d_packet_v21_pack.h
/bin/bash -c "python external/mesa/src/broadcom/cle/gen_pack_header.py \
external/mesa/src/broadcom/cle/v3d_packet_v21.xml \
> gen/STATIC_LIBRARIES/libmesa_broadcom_genxml_intermediates/broadcom/cle/v3d_packet_v21_pack.h"
Traceback (most recent call last):
  File "external/mesa/src/broadcom/cle/gen_pack_header.py", line 626, in <module>
    p = Parser(sys.argv[2])
IndexError: list index out of range

header-gen macro is already defined by Intel genxml building rules
and the existing header-gen does not have the $(PRIVATE_VER) argument,
infact the bash command line logged in the building error is missing
exactly $(PRIVATE_VER) argument

Renaming the macro as pack-header-gen in src/broadcom/Android.genxml.mk
solves the building error, another possible way is to keep the gen rules
commands expanded and not use the macros.

Fixes: 7f80a9ff

 ("vc4: Introduce XML-based packet header generation like Intel's.")
Cc: "18.2" <mesa-stable@lists.freedesktop.org>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>