Fix implicit function declarations

[ryandesign]

Fixes the following issue reported in #8:

error: implicit declaration of function 'fft_krn' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

And the additional errors I saw:

error: implicitly declaring library function 'strcmp' with type 'int (const char *, const char *)' [-Werror,-Wimplicit-function-declaration]
error: implicitly declaring library function 'memset' with type 'void *(void *, int, unsigned long)' [-Werror,-Wimplicit-function-declaration]