Fixing typos in build.json: .cpp -> .cc

This was working so far because of GNU make's magic that automatically finds the sources depending on its output, and not on its input.

This also explains why running "make" would always rebuild the plugins: because GNU make think some dependencies are phony and aren't there.
diff --git a/Makefile b/Makefile
index b43cb73..85201c0 100644
--- a/Makefile
+++ b/Makefile
@@ -2654,8 +2654,8 @@
 
 
 CPP_PLUGIN_SRC = \
-    src/compiler/cpp_plugin.cpp \
-    src/compiler/cpp_generator.cpp \
+    src/compiler/cpp_plugin.cc \
+    src/compiler/cpp_generator.cc \
 
 CPP_PLUGIN_OBJS = $(addprefix objs/$(CONFIG)/, $(addsuffix .o, $(basename $(CPP_PLUGIN_SRC))))
 
@@ -2675,8 +2675,8 @@
 
 
 RUBY_PLUGIN_SRC = \
-    src/compiler/ruby_plugin.cpp \
-    src/compiler/ruby_generator.cpp \
+    src/compiler/ruby_plugin.cc \
+    src/compiler/ruby_generator.cc \
 
 RUBY_PLUGIN_OBJS = $(addprefix objs/$(CONFIG)/, $(addsuffix .o, $(basename $(RUBY_PLUGIN_SRC))))