Prototypes were in the original C standard of 1985, before first edition of The C++ Programming Language was released and long before C++ was formalized standardized.
While the first draft was released in 1983, the first ratified C standard was in 1989.
The first edition of "The C Programming Language" (from which I first learned the language) was published in 1978, over a decade before the language was standardized.
You are correct that C89 added function prototypes. However, contrary to your implication, C borrowed the concept and syntax from C++.
There's a notable difference between C's prototypes and C++'s ... `int foo();` in C is not a prototype, whereas in C++ it's equivalent to `int foo(void);` ... C had to maintain compatibility with K&R style declarations, whereas C++ didn't.