add #pragma GCC system_header at jemalloc.h ?

[benet96]

We found posix_memalign function has conflict clang header when compiled with clang++ (issue: #1557) and a commit (https://github.com/jemalloc/jemalloc/pull/1558/files) trying to solve it.

The same declaration in stdlib.h can pass the compilation because it has -isystem. If Jemalloc has those function implementation the same as libc, we can add #pragma GCC system_header in jemalloc.h so compiler will consider jemalloc.h as system header even when user include it by ```-I

We think it's a neat patch, I used to remove the posix_memalign from jemalloc.h to resolve the confict, it works because stdlib.h has the same declaration and it will be merged to the declaration in jemalloc.h unless jemalloc.h has no declaration.