gitignore: Add precompiled headers (.gch and .pch)

We don't use precompiled headers in our builds but those files are what
you get if you invoke gcc or clang, respectively, on a .h file, e.g., to check
if it's self-contained.

Author: real-or-random

.gitignore

@@ -37,6 +37,7 @@ libtool
 *.lo
 *.o
 *~
+*.[gp]ch
 
 coverage/
 coverage.html