opencl: Fix OpenCL kernel code assertion for newer versions

With Debian package beignet-opencl-icd 1.2.1-1, Tesseract + OpenCL fails:

[DS] Profile file not available (tesseract_opencl_profile_devices.dat); performing profiling.

[DS] Device: "Intel(R) HD Graphics IvyBridge M GT2" (OpenCL) evaluation...
ASSERTION FAILED: sel.hasDoubleType()
  at file /home/geier/beignet/backend/src/backend/gen_insn_selection.cpp, function void gbe::ConvertInstructionPattern::convertDoubleToSmallInts(gbe::Selection::Opaque&, const gbe::ir::ConvertInstruction&, bool&) const, line 5269
Trace/breakpoint trap

Using a pure float expression (instead of double) fixes this issue.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

opencl/oclkernels.h

@@ -1201,7 +1201,7 @@ KERNEL(
     unsigned int word = srcData[pixelIdx];
     int output =    (rwt * ((word >> RED_SHIFT)   & 0xff) +
                      gwt * ((word >> GREEN_SHIFT) & 0xff) +
-                     bwt * ((word >> BLUE_SHIFT)  & 0xff) + 0.5);
+                     bwt * ((word >> BLUE_SHIFT)  & 0xff) + 0.5f);
     // SET_DATA_BYTE
     dstData[pixelIdx] = output;
 }