Wrap cl_ext.h and handle CL_PLATFORM_NOT_FOUND_KHR gracefully.

[maleadt]

julia> cl.platforms()
┌ Warning: No OpenCL driver JLLs were detected at the time of the first call into OpenCL.jl.
│ Only system drivers will be available.
└ @ OpenCL.cl ~/Julia/pkg/OpenCL/lib/api.jl:80
ERROR: CLError(code=-1001, CL_PLATFORM_NOT_FOUND_KHR)
Stacktrace:
 [1] platforms()
   @ OpenCL.cl ~/Julia/pkg/OpenCL/lib/platform.jl:52
 [2] top-level scope
   @ REPL[3]:1

->

julia> cl.platforms()
OpenCL.cl.Platform[]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.45%. Comparing base (9ccf867) to head (a6fa635).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #223   +/-   ##
=======================================
  Coverage   70.45%   70.45%           
=======================================
  Files           3        3           
  Lines         132      132           
=======================================
  Hits           93       93           
  Misses         39       39           

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