build-script: strip debug info from libraries and executables

One of the recent build-script refactorings has dropped the parentheses,
restore them.

rdar://problem/27083605

utils/build-script-impl

@@ -2904,7 +2904,7 @@ for host in "${ALL_HOSTS[@]}"; do
             # Strip executables, shared libraries and static libraries in
             # `host_install_destdir`.
             find "${host_install_destdir}${TOOLCHAIN_PREFIX}/" \
-              -perm -0111 -or -name "*.a" -type f -print | \
+              '(' -perm -0111 -or -name "*.a" ')' -type f -print | \
               xargs -n 1 -P ${BUILD_JOBS} $(xcrun_find_tool strip) -S
 
             { set +x; } 2>/dev/null