fix c++ witness code producer on macos with clang 14.0.0

[gr8den]

There was error while doing make command in generated cpp directory. Error example:

g++ -c fr.cpp -std=c++11 -O3 -I.
In file included from fr.cpp:1:
./fr.hpp:71:50: error: unknown type name 'uint'; did you mean 'int'?
void Fr_str2element(PFrElement pE, char const*s, uint base);
                                                 ^~~~
                                                 int
fr.cpp:56:51: error: unknown type name 'uint'; did you mean 'int'?
void Fr_str2element(PFrElement pE, char const *s, uint base) {
                                                  ^~~~
                                                  int

[nalinbhardwaj]

@clararod9 @miguelis @alrubio any reason not to merge this? this breaks on all latest macOS for all users and is a simple fix.

[miguelis]

Accepted! Thanks for your help!