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/build.json b/build.json
index 8dfd5c4..1e46540 100644
--- a/build.json
+++ b/build.json
@@ -422,8 +422,8 @@
       "c++": true,
       "secure": false,
       "src": [
-        "src/compiler/cpp_plugin.cpp",
-        "src/compiler/cpp_generator.cpp"
+        "src/compiler/cpp_plugin.cc",
+        "src/compiler/cpp_generator.cc"
       ],
       "headers": [
         "src/compiler/cpp_generator.h",
@@ -437,8 +437,8 @@
       "c++": true,
       "secure": false,
       "src": [
-        "src/compiler/ruby_plugin.cpp",
-        "src/compiler/ruby_generator.cpp"
+        "src/compiler/ruby_plugin.cc",
+        "src/compiler/ruby_generator.cc"
       ],
       "headers": [
         "src/compiler/cpp_generator.h",