package/fluidsynth: needs dynamic library

package/fluidsynth: needs dynamic library

applications, such as mpd, fail to build statically with fluidsynth
because fluidsynth does not fill Libs.Private in their pkg-config file

Because fluidsynth is a cmake-package with many dependencies, it is not
easy to fix so just add a dynamic library dependency.

Here is an extract of src/CMakeLists.txt that gives the list of
fluidsynth possible dependencies:

target_link_libraries ( libfluidsynth
${GLIB_LIBRARIES}
${GMODULE_LIBRARIES}
${LASH_LIBRARIES}
${JACK_LIBRARIES}
${ALSA_LIBRARIES}
${PULSE_LIBRARIES}
${PORTAUDIO_LIBRARIES}
${LIBSNDFILE_LIBRARIES}
${SDL2_LIBRARIES}
${DBUS_LIBRARIES}
${READLINE_LIBS}
${DART_LIBS}
${COREAUDIO_LIBS}
${COREMIDI_LIBS}
${WINDOWS_LIBS}
${MidiShare_LIBS}
${OpenSLES_LIBS}
${OBOE_LIBS}
${LIBFLUID_LIBS}
${LIBINSTPATCH_LIBRARIES}
)

Fixes:
 - http://autobuild.buildroot.org/results/ec9dd2903359b9bf6b15c8cb69e732f8cb6c4d39

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