Add missing span-locations feature for proc-macro2 in line_count

[ouuan]

I faced compilation errors without this feature flag. This feature has been required since proc-macro2 1.0.25, so this should be already broken when the line_count tool was first added. I'm wondering why this was not discovered earlier 🤔️

[ouuan]

Oh, I just found

verus/source/tools/line_count/.cargo/config.toml

Lines 1 to 2 in 8e7e368

	[build]
	rustflags = "--cfg proc_macro_span --cfg span_locations"

Somehow my cargo is not reading it.

[ouuan]

Oh, I have the following in my ~/.cargo/config.toml to use mold. The target-specific setting overrode the project local setting 🥲

[target.x86_64-unknown-linux-gnu]
linker = "/usr/bin/clang"
rustflags = [
    "-Clink-arg=-fuse-ld=/usr/bin/mold",
]

reference: rust-lang/cargo#5376 (comment)