Use hidden visibility as in the LO code here, too

Otherwise we get tons of (as such, in our case harmless) warnings from
the linker about mismatches.

Change-Id: I826d9e065bae59cdd213131163b31b2099806dd3

ios/shared/ios_sharedlo.xcodeproj/project.pbxproj

@@ -545,7 +545,7 @@
 					"MLO_APP_ROLE=LO_APP",
 					"DEBUG=1",
 				);
-				GCC_SYMBOLS_PRIVATE_EXTERN = NO;
+				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES;
 				GCC_WARN_UNINITIALIZED_AUTOS = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
@@ -582,6 +582,7 @@
 				GCC_C_LANGUAGE_STANDARD = gnu99;
 				GCC_DYNAMIC_NO_PIC = NO;
 				GCC_PREPROCESSOR_DEFINITIONS = "MLO_APP_ROLE=LO_APP";
+				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES;
 				GCC_WARN_UNINITIALIZED_AUTOS = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;
@@ -660,7 +661,7 @@
 					"MLO_APP_ROLE=TILE_TESTER",
 					"DEBUG=1",
 				);
-				GCC_SYMBOLS_PRIVATE_EXTERN = NO;
+				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
 				GCC_WARN_ABOUT_RETURN_TYPE = YES;
 				GCC_WARN_UNINITIALIZED_AUTOS = YES;
 				GCC_WARN_UNUSED_VARIABLE = YES;