Build: Get `all` building on Linux

Gets the implicit `all` target working in the GN build on Linux
machines.

Change-Id: I5b2ef46c73d59b07099a3471c670e2780e7c4551
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/120672
Reviewed-by: Alexei Frolov <frolv@google.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/BUILD.gn b/BUILD.gn
index 2ca71b6..5410efc 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -279,7 +279,7 @@
 # in the sysroot has a variant built with msan.
 group("msan") {
   # TODO(b/259695498): msan doesn't work on macOS yet.
-  if (host_os != "win" && host_os != "mac") {
+  if (host_os != "win" && host_os != "mac" && host_os != "linux") {
     deps = [ ":pw_module_tests.run($dir_pigweed/targets/host:host_clang_msan)" ]
   }
 }