fix restrict usage in VLC include files

Right now the code uses AC_C_RESTRICT to pick a restrict value that works in C and C++. It's fine with gcc and clang that have __restrict or similar keyword that is understood by both their C and C++ compiler.

But a pure C11 compiler that doesn't have those will en up having no define because it supports the restrict keyword natively. The same compiler when compiling C++ code will have no idea what this restrict keyword is in our headers.

This is easy to test but just commenting the AC_C_RESTRICT line in configure.ac. It will just use the restrict keyword of gcc/clang when compiling C code, and will fail when a header used by C++ modules is used. In other words the AC_C_RESTRICT is only there to find a restrict variant that would also work with C++.

The AC_C_RESTRICT documentation is even bogus as it claims "If the C compiler recognizes the restrict keyword, don't do anything.". But this is not what is done with either gcc or clang, and they do recognize the restrict keyword.