linux process: silence GCC switch coverage warning

Add missing entry for eExecMessage message type to silence GCC switch coverage
warning.

llvm-svn: 213470
diff --git a/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp b/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
index 7e6f319..9e5f844 100644
--- a/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
+++ b/lldb/source/Plugins/Process/Linux/ProcessMonitor.cpp
@@ -1844,6 +1844,8 @@
 
         switch (message.GetKind())
         {
+            case ProcessMessage::eExecMessage:
+                llvm_unreachable("unexpected message");
             case ProcessMessage::eAttachMessage:
             case ProcessMessage::eInvalidMessage:
                 break;