Cycles: Fix/workaround for gcc-7.2

For some reason when building with gcc-7.2 (which is default
in previous Ubuntu LTS) the guarded allocator is not being
properly instantiated.

Doesn't happen with newer version of gcc-7 which is 7.3, and
also doesn't happen with gcc-6 and gcc-8.

Would be nice to know what is wrong, but for the time being
committing workaround which keeps Blender users happy.

intern/cycles/util/util_ies.cpp

@@ -21,6 +21,13 @@
 
 CCL_NAMESPACE_BEGIN
 
+// NOTE: For some reason gcc-7.2 does not instantiate this versio of allocator
+// gere (used in IESTextParser). Works fine for gcc-6, gcc-7.3 and gcc-8.
+//
+// TODO(sergey): Get to the root of this issue, or confirm this i a compiler
+// issue.
+template class GuardedAllocator<char>;
+
 bool IESFile::load(ustring ies)
 {
 	clear();