package/postgresql: fix snprintf() detection

https://git.busybox.net/buildroot/commit/package/postgresql/postgresql.mk?id=3d3f70b567718ae308b5704ddd7d107c4849309e
forced postgresql not to use its own snprintf() implementation which
needs libm. This fix got broken after this upstream commit

http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=ce486056ecd28050f367894a2b5aad3656d37511#patch1

changed the variable name from
  pgac_cv_snprintf_long_long_int_format
to
  pgac_cv_snprintf_long_long_int_modifier

leading to problems in the rsyslog package while detecting postgresql
support.

Fixes
http://autobuild.buildroot.net/results/1cd/1cda32983fdf9522e4a9d48cc1304cc1451771a9/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>