Move extern crate proc_macro into each possible lib.rs

This makes it slightly more convenient to use the following as a
Reindeer fixup for those that prefer to build from source:

    extra_mapped_srcs = { "src/lib_from_source.rs" = "src/lib.rs" }

    [platform_fixups.'cfg(all(target_arch = "x86_64", target_os = "linux", target_env = "gnu"))']
    extra_deps = [":proc-macro2", ":quote", ":syn"]

as opposed to checking in a whole new file containing the `extern crate
proc_macro` + `include!("lib_from_source.rs")`.

precompiled/serde_derive/src/lib.rs

@@ -16,8 +16,6 @@
 #![doc(html_root_url = "https://docs.rs/serde_derive/1.0.175")]
 #![allow(unknown_lints, bare_trait_objects)]
 
-extern crate proc_macro;
-
 #[cfg(not(all(target_arch = "x86_64", target_os = "linux", target_env = "gnu")))]
 include!("lib_from_source.rs");