[scudo] Get rid of builtin-declaration-mismatch warnings

Summary:
The C interceptors were using `SIZE_T` defined in the interception library as
a `__sanitizer::uptr`. On some 32-bit platforms, this lead to the following
warning:
```
warning: declaration of ‘void* malloc(SIZE_T)’ conflicts with built-in declaration ‘void* malloc(unsigned int)’ [-Wbuiltin-declaration-mismatch]
INTERCEPTOR_ATTRIBUTE void *malloc(SIZE_T size) {
```
`__sanitizer::uptr` is indeed defined as an `unsigned long` on those.

So just include `stddef.h` and use `size_t` instead.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48885

llvm-svn: 336221