opencl: fix beignet blendop warning

Beignet 0.9.2 built against LLVM 3.4 produces the following warning while compiling our OpenCL kernels:

[opencl_init] compiling program `blendop.cl' ..
[opencl_fopen_stat] could not open file `/home/pmjdebruijn/.cache/darktable/cached_kernels_for_IntelRHDGraphicsHaswellUltrabookGT2Mobile/blendop.cl.bin'!
[opencl_load_program] could not load cached binary program, trying to compile source
[opencl_load_program] successfully loaded program from `/usr/share/darktable/kernels/blendop.cl'
[opencl_build_program] successfully built program
[opencl_build_program] BUILD STATUS: 0
BUILD LOG:
/tmp/47AWjM.cl:166:17: warning: logical not is only applied to the left hand side of this comparison
/tmp/47AWjM.cl:166:17: note: add parentheses after the '!' to evaluate the comparison first
/tmp/47AWjM.cl:166:17: note: add parentheses around left hand side expression to silence this warning
/tmp/47AWjM.cl:235:17: warning: logical not is only applied to the left hand side of this comparison
/tmp/47AWjM.cl:235:17: note: add parentheses after the '!' to evaluate the comparison first
/tmp/47AWjM.cl:235:17: note: add parentheses around left hand side expression to silence this warning