gcc/rust/Make-lang.in: add missing rust compiler driver

When building gccrs with Buildroot toolchain infrastructure, the gccrs
compiler driver is missing when intalling gcc.

This is due to missing depedency on gccrs$(exeext) in rust.all.cross
target.

Signed-off-by: Romain Naour <[email protected]>
---
We probably needs gccrs-cross target like other supported languages.

Author: RomainNaour

gcc/rust/Make-lang.in

@@ -140,7 +140,7 @@ check_rust_parallelize = 10
 # was specified as a configure option.
 rust.srcextra:
 
-rust.all.cross:
+rust.all.cross: gccrs$(exeext)
 
 # idk what this does but someone used it
 rust.start.encap: gccrs$(exeext)