ocl: Don't use 'inline' attribute on functions

In C99 'inline' is not a hint to the compiler to inline the function,
it is an attribute that affects the linkage of the function.  'inline'
functions are required to have a definition in a different compiliation
unit, so compilers are free to delete 'inline' functions if they want to.

This issue can be seen in Clang when compiling at -O0.  Clang
will sometimes delete 'inline' functions which creates an invalid
program.

Issue 3746: http://code.opencv.org/issues/3746