package/libsvgtiny: fix parallel build

Fix previous commit[1] which purpose was to fix parallel build. It
didn't work since it assigned $(MAKE1) to LIBSVGTINY_MAKE, but this is a
generic-package and building is done using $(MAKE), then LIBSVGTINY_MAKE
was ignored. Let's substitute instead $(MAKE) with $(MAKE1) in
LIBSVGTINY_BUILD_CMDS.

[1]:
https://git.buildroot.net/buildroot/commit/?id=26d67a2599d6c88facd5178de853fa355244e7c2

Fixes:
http://autobuild.buildroot.net/results/67d/67d341c0cc272323d6e231a20796a6848c21d760/



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[yann.morin.1998@free.fr:
  - use $(MAKE1) in all three step
  - move comment out of the define
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>