[scudo] Fix for the Scudo interface function scope

Summary:
A forgotten include in `scudo_allocator.cpp` made the symbol only local :/

Before:
```
nm ./lib/clang/7.0.0/lib/linux/libclang_rt.scudo-i686-android.so | grep rss
00024730 t __scudo_set_rss_limit
```
After:
```
nm ./lib/clang/7.0.0/lib/linux/libclang_rt.scudo-i686-android.so | grep rs
00024760 T __scudo_set_rss_limit
```
And we want `T`!

This include also means that we can get rid of the `extern "C"` in the C++
file, the compiler does fine without it (note that this was already the case
for all the `__sanitizer_*` interface functions.

Reviewers: alekseyshl, eugenis

Reviewed By: eugenis

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42199

llvm-svn: 322782