Possible Windows build fix.

This just tries to convince MSVC that we're not going to use |nprimes|
without initialising it first:

tool\genrsa.cc(63) : warning C4701: potentially uninitialized local
variable 'nprimes' used

Change-Id: If8a68ad4fe2c2fb7a8073b7ba43d540467ddf8f8
1 file changed