Fix type conversion

Fixes the following build error with gcc 6.x:

protocol.cc:27:40: error: narrowing conversion of ''\37777777776'' from 'char' to 'uchar {aka unsigned char}' inside { } [-Wnarrowing]
 static uchar eof_buff[1]= { (char) 254 }; /* Marker for end of fields */

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/server-tools/instance-manager/protocol.cc
===================================================================
--- a/server-tools/instance-manager/protocol.cc
+++ b/server-tools/instance-manager/protocol.cc
@@ -24,7 +24,7 @@
 #include <m_string.h>
 
 
-static uchar eof_buff[1]= { (char) 254 }; /* Marker for end of fields */
+static uchar eof_buff[1]= { (uchar) 254 }; /* Marker for end of fields */
 static const char ERROR_PACKET_CODE= (char) 255;