Description:
------------
Add the gmp_nextprime function to ext/gmp. I need it for    
my cryptographically secure random number generator and    
it's probably pretty useful to others too (espescially  
because I can then release my code properly :).  
  
http://www.hot.ee/stepz/Math_Random/gmp.patch is the 
(really simple) patch. 

Reproduce code:
---------------
if (function_exists("gmp_nextprime")) {
echo gmp_strval(gmp_nextprime(1000));
} else {
echo "No function";
}

Expected result:
----------------
1009 

Actual result:
--------------
No function