package/nginx-modsecurity: fix static build

Fix static build which has been enabled since bump of libmodsecurity to
version 3.0.5 in commit 464d0be3

Fixes:
 - http://autobuild.buildroot.org/results/e5a9eb8448980f1c5cafe97180b7d1f48ddf02ca



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>