hidapi: fix build with toolchains lacking C++ support

hidapi currently doesn't build on non-C++ capable toolchains due to
hidtest being a C++ source file, even if it in facts contains only C
code. This commit adds a patch that fixes this in the hidapi source
code.

Fixes:

  http://autobuild.buildroot.org/results/6ce/6ce0a4b8c7acb857005350a57c313b493bc6e2b7/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>