build: use FindJNI to locate required headers

We use `jni.h` for the android support library. Setup the search in
CMake to allow us to find the header and include the header search path
properly.

CMakeLists.txt

@@ -6,6 +6,7 @@ project(swift-firebase
 if(ANDROID)
   enable_language(C)
   include(FindJava)
+  include(FindJNI)
   include(UseJava)
 endif()
 
@@ -86,6 +87,8 @@ target_link_libraries(FirebaseCore PRIVATE
 if(ANDROID)
   target_link_libraries(FirebaseCore PUBLIC
     FirebaseAndroidJNI)
+  target_link_libraries(FirebaseCore PRIVATE
+    JNI::JNI)
 endif()
 
 add_library(FirebaseAuth SHARED