postgresql: mark as not available for static linking

postgresql currently does not build in BR2_STATIC_LIBS=y
configurations, and since there is little interest in using such a big
piece of software in statically linked configuration, this commit
makes postgresql depends on !BR2_STATIC_LIBS.

Fixes:

  http://autobuild.buildroot.net/results/f9ed96d22e91cdba9ad92c4d4ea52e422bf1f1c9/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>