[Bug] Build failure on musl

[vadorovsky]

error[E0425]: cannot find function `malloc_trim` in crate `libc`
   --> src/backend/mod.rs:115:19
    |
115 |             libc::malloc_trim(0);
    |                   ^^^^^^^^^^^ not found in `libc`

For more information about this error, try `rustc --explain E0425`.
error: could not compile `cosmic-store` (bin "cosmic-store") due to 1 previous error

malloc_trim is a GNU extension not available on musl. I'll think of an alternative and come back with a PR.