libopenmpt: add upstream patch to fix build with gcc-6

Signed-off-by: Stephan Raue <stephan@openelec.tv>
(cherry picked from commit 60560259bdc2b7883b76bb08b9af3c7fde12b169)

packages/addons/depends/audio/libopenmpt/patches/libopenmpt-bug-769.patch

@@ -0,0 +1,14 @@
+Index: soundlib/tuning.cpp
+===================================================================
+--- a/soundlib/tuning.cpp	(revision 6294)
++++ b/soundlib/tuning.cpp	(working copy)
+@@ -375,7 +375,8 @@
+ 	if(!IsOfType(TT_GROUPGEOMETRIC)) return 0;
+ 
+ 	if(note >= 0) return note % GetGroupSize();
+-	else return (GetGroupSize() -  (abs(note) % GetGroupSize())) % GetGroupSize();
++	// static_cast<int>(note) because g++-6 template-disables std::abs overloads for 8bit and 16bit integer types via std::enable_if.
++	else return (GetGroupSize() - (std::abs(static_cast<int>(note)) % GetGroupSize())) % GetGroupSize();
+ }
+ 
+