Avoid superfluous compiler warnings

GCC warns about uninitialized local variables in cases where no
initialization is needed, in particular in the overloads of the
'Capability::call()' function. Prior this patch, we dealt with those
warnings by using an (unreliable) GCC pragma or by disabling the
particular warning altogether (which is a bad idea). This patch removes
the superfluous warnings by telling the compiler that the variable in
question is volatile.

os/src/app/chroot/target.mk

@@ -2,18 +2,3 @@ TARGET   = chroot
 REQUIRES = linux
 SRC_CC   = main.cc
 LIBS     = cxx env server lx_hybrid
-
-#
-# XXX find a way to remove superfluous warning:
-#
-# base/include/util/token.h: In constructor ‘Genode::Config::Config()’:
-# base/include/util/token.h:69:67: warning: ‘ret’ may be used uninitialized in
-#                                            this function [-Wuninitialized]
-# base/include/base/capability.h:196:62: note: ‘ret’ was declared here
-# base/include/util/token.h:100:68: warning: ‘prephitmp.1897’ may be used
-#                                            uninitialized in this function
-#                                            [-Wuninitialized]
-# os/include/os/config.h:42:4: note: ‘prephitmp.1897’ was declared here
-#
-CC_WARN  = -Wall -Wno-uninitialized
-