Fix linking with BUILD_SHARED_LIBS=ON default in EAPI-7

--- a/src/common_netplay/CMakeLists.txt
+++ b/src/common_netplay/CMakeLists.txt
@@ -1,4 +1,4 @@
 
 if(NOT NO_NETWORK)
-    add_library(NetplayFiles platform_enet/NetPeerENet.cpp)
+    add_library(NetplayFiles STATIC platform_enet/NetPeerENet.cpp)
 endif()
--- a/src/common/CMakeLists.txt
+++ b/src/common/CMakeLists.txt
@@ -39,4 +39,4 @@
     list(APPEND COMMON_SRCS src/savepng.cpp)
 endif()
 
-add_library(CommonFiles ${COMMON_SRCS})
+add_library(CommonFiles STATIC ${COMMON_SRCS})