sys: enable parallel builds

BurntSushi · BurntSushi · commit b0b0d81a163f · 2020-08-19T18:52:11.000-04:00
This causes the C compiler to build in parallel. Kudos to @joshtriplett for the tip!
diff --git a/pcre2-sys/Cargo.toml b/pcre2-sys/Cargo.toml
@@ -16,5 +16,5 @@ edition = "2018"
 libc = "0.2"
 
 [build-dependencies]
-cc = "1"
+cc = { version = "1", features = ["parallel"] }
 pkg-config = "0.3.13"
