tcpdump requires a C compiler that supports prototypes, as they're used

tcpdump requires a C compiler that supports prototypes, as they're used
throughout it; get rid of the use of the "__P()" macro, as, at least on
some platforms with some compilers, that macro doesn't expand to its
argument, causing the functions not to be declared with function
prototypes, which then causes the compiler to warn about the prototype
declaration and the function definition disagreeing.