Patch based on the upstream commit described below
with paths updated to reflect the Debian rust-onig-sys
package.

commit c88ab90653c416b63ec994c3883579a15a3bba66
Author: Will Speak <lithiumflame@gmail.com>
Date:   Sun Nov 1 12:14:44 2020 +0000

    Add More Pointer Sizes to Config Header
    
    This is needed for 32 bit LP32 targets to realise they are 32 bit.

Index: onig-sys/build.rs
===================================================================
--- onig-sys.orig/build.rs
+++ onig-sys/build.rs
@@ -116,6 +116,7 @@ fn compile() {
             #define SIZEOF_LONG {0}
             #define SIZEOF_VOIDP {0}
             #define SIZEOF_LONG_LONG 8
+
         ",
                 if bits == "64" { "8" } else { "4" }
             ),