fuzz: fix build error

[manunio]

Description

wasmer's oss-fuzz build was failing with following error:

error: variable does not need to be mutable
   --> lib/compiler-llvm/src/compiler.rs:279:25
    |
279 |                     for mut reloc in &mut custom_section.relocations {
    |                         ----^^^^^
    |                         |
    |                         help: remove this `mut`
    |
note: the lint level is defined here
   --> lib/compiler-llvm/src/lib.rs:4:5
    |
4   |     unused_mut,
    |     ^^^^^^^^^^

error: variable does not need to be mutable
   --> lib/compiler-llvm/src/compiler.rs:291:29
    |
291 |                         for mut reloc in &mut custom_section.relocations {
    |                             ----^^^^^
    |                             |
    |                             help: remove this `mut`

error: variable does not need to be mutable
   --> lib/compiler-llvm/src/compiler.rs:306:21
    |
306 |                 for mut reloc in &mut compiled_function.compiled_function.relocations {
    |                     ----^^^^^
    |                     |
    |                     help: remove this `mut`

error: could not compile `wasmer-compiler-llvm` (lib) due to 3 previous errors

This pr attempts to fix that by removing unused mut.
Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=58561

[ptitSeb]

Thanks for the fix!