Restrict export of libjpeg symbols from libjavafx_iio.so

libjavafx_iio.so contains a copy of libjpeg7 (soon to be libjpeg9)
and since we are exporting all the libjpeg symbols these may
clash with some other libjpeg on a system.
We should restrict the export to just the JNI interfaces as the
JDK does in similar situations.
 
See https://bugs.openjdk.java.net/browse/JDK-8200178 and
http://mail.openjdk.java.net/pipermail/build-dev/2018-March/021306.html

The gist of the fix there is to add a JNIEXPORT directive to the declarations of the JNI functions in the JDK's own source files
which are compiled into the library and to update the makefile
so that all symbols not explicitly exported are internal.