package/libpri: add -fPIC

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=11961

[Retrieved (and updated to keep line under 80 characters) from:
https://bugs.buildroot.org/show_bug.cgi?id=11961

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