c99_compat.h: Don't try to use 'restrict' in C++ code

c99_compat.h: Don't try to use 'restrict' in C++ code

Fixes build failures on Solaris in C++ files using gcc:

../src/util/u_math.h:628:41: error: expected ‘,’ or ‘...’ before ‘dest’
  628 | util_memcpy_cpu_to_le32(void * restrict dest, const void * restrict src, size_t n)
      |                                         ^~~~
../src/util/u_math.h: In function ‘void* util_memcpy_cpu_to_le32(void*)’:
../src/util/u_math.h:641:18: error: ‘dest’ was not declared in this scope
  641 |    return memcpy(dest, src, n);
      |                  ^~~~
../src/util/u_math.h:641:24: error: ‘src’ was not declared in this scope
  641 |    return memcpy(dest, src, n);
      |                        ^~~
../src/util/u_math.h:641:29: error: ‘n’ was not declared in this scope; did you mean ‘yn’?
  641 |    return memcpy(dest, src, n);
      |                             ^
      |                             yn

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>