remove unnecessary extern C on panic handler to fix not-ffi-safe warning

[cmsd2]

1.40.0-nightly has added a warning about extern functions that are not ffi-safe.
the panic handler is unnecessarily marked as extern and trips this warning.
this pull request removes the extern decleration to fix the warning.

Building bootloader
   Compiling bootloader v0.8.2 (/Users/cmsd2/.cargo/registry/src/github.com-1ecc6299db9ec823/bootloader-0.8.2)
warning: `extern` fn uses type `core::panic::PanicInfo`, which is not FFI-safe
   --> src/main.rs:368:31
    |
368 | pub extern "C" fn panic(info: &PanicInfo) -> ! {
    |                               ^^^^^^^^^^ not FFI-safe

Fixes #84

[phil-opp]

Thanks a lot for the pull request!

There seems to be a small problem with the "Check Formatting" CI job, but this has nothing to do with your changes. I will fix it and merge this PR afterwards.

[phil-opp]

CI should be fixed now.

bors r+

[phil-opp]

There seems to be a problem with the bors bot: bors-ng/bors-ng#806

Merging manually

[phil-opp]

Released as version 0.8.3. Thanks again for the pull request!