AIX: Install ExportImportList script with execute permission

This script was added by commit 0f150b69d3 (AIX: Explicitly compute
shared object exports for both XL and GNU, 2019-07-11,
v3.16.0-rc1~418^2~2) but does not have a `.sh` extension so our existing
install rules neglect to give it execute permission.  Our test suite
works on AIX in the build tree but the script is broken without execute
permission on installation.

Fixes: #20520
1 file changed