package/mupdf: fix building shared libraries
By default mupdf generates static libraries, shared libraries must be
enabled explicitely.
Also, when building shared libraries, mupdf's Makefile properly passes
-fPIC, so adding it manually to MUPDF_CFLAGS is not needed.
Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>