Fix arch linux makedepends

[snowinmars]

#690 fails without these dependencies.

I set them in Dockerfile manually, but I think this pr is the right way.

gcc is required for proc-macro2.
pkg-config is required for rust-openssl.

[codecov]

Codecov Report

Merging #691 (7c037fa) into master (ac94ca4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #691   +/-   ##
=======================================
  Coverage   46.35%   46.35%           
=======================================
  Files         133      133           
  Lines       60343    60343           
=======================================
+ Hits        27973    27974    +1     
+ Misses      32370    32369    -1     

Impacted Files	Coverage Δ
src/bin/tectonic.rs	71.42% <0.00%> (+0.75%)	⬆️

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[pkgw]

Thanks!