fltk: Fix license warning

WARNING: fltk: No generic license file exists for: FLTK in any provider

Add license

Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/licenses/FLTK

@@ -0,0 +1,43 @@
+                             FLTK License
+                           December 11, 2001
+
+The FLTK library and included programs are provided under the terms
+of the GNU Library General Public License (LGPL) with the following
+exceptions:
+
+    1. Modifications to the FLTK configure script, config
+       header file, and makefiles by themselves to support
+       a specific platform do not constitute a modified or
+       derivative work.
+
+       The authors do request that such modifications be
+       contributed to the FLTK project - send all
+       contributions to "fltk-bugs@fltk.org".
+
+    2. Widgets that are subclassed from FLTK widgets do not
+       constitute a derivative work.
+
+    3. Static linking of applications and widgets to the
+       FLTK library does not constitute a derivative work
+       and does not require the author to provide source
+       code for the application or widget, use the shared
+       FLTK libraries, or link their applications or
+       widgets against a user-supplied version of FLTK.
+
+       If you link the application or widget to a modified
+       version of FLTK, then the changes to FLTK must be
+       provided under the terms of the LGPL in sections
+       1, 2, and 4.
+
+    4. You do not have to provide a copy of the FLTK license
+       with programs that are linked to the FLTK library, nor
+       do you have to identify the FLTK license in your
+       program or documentation as required by section 6
+       of the LGPL.
+
+       However, programs must still identify their use of FLTK.
+       The following example statement can be included in user
+       documentation to satisfy this requirement:
+
+           [program/widget] is based in part on the work of
+           the FLTK project (http://www.fltk.org).