dpctl_capi does not compile with C compiler

[oleksandr-pavlyk]

18:15:03  <elided>/lib/python3.10/site-packages/dpctl/include/dpctl_capi.h:31:32: \
                                    error: expected identifier or '(' before string constant
18:15:03   #define CYTHON_EXTERN_C extern "C"
18:15:03                                  ^

This was introduced while transitioning to Cython 3.0 in gh-1302.

This define was added unconditionally, while it should have only been added for C++ compilers.