Java bindings fail to build on windows in CI

[anka-213]

See for example this build: https://github.com/anka-213/gf-core/runs/2996545839?check_suite_focus=true

This is the error message we get:

libtool  --tag=CC --mode=compile gcc  -g -O -c -I "/c/Program Files/Java/jdk8u275-b01/include" -I "/c/Program Files/Java/jdk8u275-b01/include/win32" -I "/mingw64/include" -D__int64=int64_t  -std=c99 -shared jpgf.c -o jpgf.lo
libtool: compile:  gcc -g -O -c -I "/c/Program Files/Java/jdk8u275-b01/include" -I "/c/Program Files/Java/jdk8u275-b01/include/win32" -I /mingw64/include -D__int64=int64_t -std=c99 jpgf.c  -DDLL_EXPORT -DPIC -o .libs/jpgf.o
jpgf.c:6:10: fatal error: jni.h: No such file or directory
    6 | #include <jni.h>
      |          ^~~~~~~
compilation terminated.
make: *** [Makefile:33: jpgf.lo] Error 1

[johnjcamilleri]

Ugh, I thought I had fixed this. Investigating here.