Cleanup: Add missing include for parallel reduction

The parallel reduction file didn't include its own header, which can
yield "no previous declaration" warnings. This patch includes the header
to fix the warning.

source/blender/compositor/realtime_compositor/CMakeLists.txt

@@ -2,6 +2,7 @@
 
 set(INC
   .
+  ./algorithms
   ../../blenkernel
   ../../blenlib
   ../../gpu

source/blender/compositor/realtime_compositor/algorithms/intern/algorithm_parallel_reduction.cc

@@ -12,6 +12,8 @@
 #include "COM_context.hh"
 #include "COM_utilities.hh"
 
+#include "COM_algorithm_parallel_reduction.hh"
+
 namespace blender::realtime_compositor {
 
 /* Reduces the given texture into a single value and returns it. The return value should be freed