Revert .cu -> .cxx change for option_parser TU.

This change introduced a strange bug on GCC where a stack object is changed
seemingly randomly.

In `option_parser::update_axis`, the `flag` variable's `data()` pointer
is overwritten to point at garbage memory shortly after parsing. Nothing
related to this object is being updated when the corruption occurs.

This has been observed on gcc7+, but cannot be reproduced on MSVC.

Reverting the `option_parser.cxx` TU to be a CUDA object works
around this.

Fixes issue #36.

nvbench/CMakeLists.txt

@@ -12,7 +12,7 @@ set(srcs
   int64_axis.cxx
   markdown_printer.cu
   named_values.cxx
-  option_parser.cxx
+  option_parser.cu
   printer_base.cxx
   printer_multiplex.cxx
   runner.cxx