FSEU16_MAX_MEMORY_USAGE is defined at fseU16.c not a fseU16.h

When I test the FSE-16bit version, I found some issue about the `FSE_MAX_MEMORY_USAGE`.
This is the code I tested.

```c
#include
#include
#include
#include "fseU16.h"

int main() {
// Change the FSEU16_MAX_SYMBOL_VALUE to 4095
int n = 10000, max = 4095;
uint16_t *ip = malloc(sizeof(uint16_t) * n);
uint8_t *op = malloc(sizeof(uint8_t) * 2 * n);

for (int i=0; i