what library you have linked your binary against?
Show the compiler line please.

This should be solved by adding the unix3r library.

For example, file test.c

#include <sys/statfs.h>
#include <stdio.h>

int main(void){
	struct statfs buffer;
	int ret;
	
	ret = statfs("/", &buffer, 0, 0);
	
	return 0;
}

can be compiled with the next command:

# cc test.c -lunix3r -o test