Add visibility hidden in example Makefiles

Author: jonatanklosko

example/Makefile

@@ -2,7 +2,7 @@ PRIV_DIR := $(MIX_APP_PATH)/priv
 NIF_PATH := $(PRIV_DIR)/libexample.so
 C_SRC := $(shell pwd)/c_src
 
-CPPFLAGS := -shared -fPIC -std=c++17 -Wall -Wextra
+CPPFLAGS := -shared -fPIC -fvisibility=hidden -std=c++17 -Wall -Wextra
 CPPFLAGS += -I$(ERTS_INCLUDE_DIR) -I$(FINE_INCLUDE_DIR)
 
 ifdef DEBUG

test/Makefile

@@ -2,7 +2,7 @@ PRIV_DIR := $(MIX_APP_PATH)/priv
 NIF_PATH := $(PRIV_DIR)/libfinest.so
 C_SRC := $(shell pwd)/c_src
 
-CPPFLAGS := -shared -fPIC -std=c++17 -Wall -Wextra
+CPPFLAGS := -shared -fPIC -fvisibility=hidden -std=c++17 -Wall -Wextra
 CPPFLAGS += -I$(ERTS_INCLUDE_DIR) -I$(FINE_INCLUDE_DIR)
 # We want to eliminate all warnings, so the end user doesn't see any.
 CPPFLAGS += -Werror