Hide unexported symbols

We now have:
$ nm -g --defined-only libccid.so
000000000000bf90 T IFDHCloseChannel
000000000000ea40 T IFDHControl
000000000000bf80 T IFDHCreateChannel
000000000000bf70 T IFDHCreateChannelByName
0000000000010ad0 T IFDHGetCapabilities
0000000000010880 T IFDHICCPresence
000000000000c1b0 T IFDHPowerICC
000000000000c030 T IFDHSetCapabilities
000000000000c5d0 T IFDHSetProtocolParameters
000000000000e260 T IFDHTransmitToICC

Thanks to Maksim Ivanov for the idea
" Mark suitable variables and functions static #147 "
#147

meson.build

@@ -18,6 +18,9 @@ conf_data = configuration_data({
   'USE_COMPOSITE_AS_MULTISLOT' : get_option('composite-as-multislot'),
   })
 
+# global arguments
+add_global_arguments('-fvisibility=hidden', language : 'c')
+
 # tests for functions
 compiler = meson.get_compiler('c')
 if compiler.has_function('strlcpy')