Add Missing LGPL_EXCEPTION.txt

Fixes #671

src/3rdparty/LGPL_EXCEPTION.txt

@@ -0,0 +1,22 @@
+Digia Qt LGPL Exception version 1.1
+
+As an additional permission to the GNU Lesser General Public License version
+2.1, the object code form of a "work that uses the Library" may incorporate
+material from a header file that is part of the Library. You may distribute
+such object code under terms of your choice, provided that:
+(i) the header files of the Library have not been modified; and
+(ii) the incorporated material is limited to numerical parameters, data
+structure layouts, accessors, macros, inline functions and
+templates; and
+(iii) you comply with the terms of Section 6 of the GNU Lesser General
+Public License version 2.1.
+
+Moreover, you may apply this exception to a modified version of the Library,
+provided that such modification does not involve copying material from the
+Library into the modified Library's header files unless such material is
+limited to (i) numerical parameters; (ii) data structure layouts;
+(iii) accessors; and (iv) small macros, templates and inline functions of
+five lines or less in length.
+
+Furthermore, you are not required to apply this additional permission to a
+modified version of the Library.