[PATCH] gas: Fix a testcase broken by new ZSTD support

[Cary Coutant]

> The commit 1369522f36eece1b37139a81f7f2139ba3915172 ("Recognize the new ELF
> compression type for ZSTD.") added the new ELF compression type but it
> accidentally broke a GAS testcase.  Since 2 is now a valid compression type
> ELFCOMPRESS_ZSTD, the ".word 2" line in section10.s which expects an
> **unknown** compression type needs to be changed to another unknown value.

Mea culpa! For such a trivial change, it didn't occur to me that it
might have a downstream effect, and I ran only the binutils tests.
Sorry, I should've run them all.

-cary