fixed gcc warnings 'integer constant is so large that it is unsigned'

An easy one, here is a .proto file to reproduce:

message test {
        optional uint64 test = 1 [default=18446744073709551615];
}

Without the fix compiler complains like this:

$ gcc -c test.pb-c.c 
test.pb-c.c: In function ‘test__init’:
test.pb-c.c:13:28: warning: integer constant is so large that it is unsigned [enabled by default]
test.pb-c.c:13:3: warning: this decimal constant is unsigned only in ISO C90 [enabled by default]
test.pb-c.c: At top level:
test.pb-c.c:53:51: warning: integer constant is so large that it is unsigned [enabled by default]
test.pb-c.c:53:1: warning: this decimal constant is unsigned only in ISO C90 [enabled by default]