patch 9.0.0738: cannot suppress completion "scanning" messages

Problem:    Cannot suppress completion "scanning" messages.
Solution:   Add the "C" flag in 'shortmess'. (Bjorn Linse, closes #11354)
diff --git a/src/option.h b/src/option.h
index 0e98824..19e2a78 100644
--- a/src/option.h
+++ b/src/option.h
@@ -265,11 +265,12 @@
 #define SHM_ATTENTION	'A'		// no ATTENTION messages
 #define SHM_INTRO	'I'		// intro messages
 #define SHM_COMPLETIONMENU  'c'		// completion menu messages
+#define SHM_COMPLETIONSCAN  'C'		// completion scanning messages
 #define SHM_RECORDING	'q'		// short recording message
 #define SHM_FILEINFO	'F'		// no file info messages
 #define SHM_SEARCHCOUNT  'S'		// search stats: '[1/10]'
 #define SHM_POSIX       "AS"		// POSIX value
-#define SHM_ALL		"rmfixlnwaWtToOsAIcqFS" // all possible flags for 'shm'
+#define SHM_ALL		"rmfixlnwaWtToOsAIcCqFS" // all possible flags for 'shm'
 
 // characters for p_go:
 #define GO_TERMINAL	'!'		// use terminal for system commands