Fix warnings for mongoc-b64

Implementation in headers is bad.  Separate out the implementation and
file scoped statics into a .c and pull the necessary function definitions
out into the associated -private.h

Author: hanumantmk

CMakeLists.txt

@@ -59,6 +59,7 @@ set (SOURCES
    ${SOURCE_DIR}/src/mongoc/mongoc-array.c
    ${SOURCE_DIR}/src/mongoc/mongoc-bulk-operation.c
    ${SOURCE_DIR}/src/mongoc/mongoc-buffer.c
+   ${SOURCE_DIR}/src/mongoc/mongoc-b64.c
    ${SOURCE_DIR}/src/mongoc/mongoc-client.c
    ${SOURCE_DIR}/src/mongoc/mongoc-client-pool.c
    ${SOURCE_DIR}/src/mongoc/mongoc-cluster.c

src/mongoc/Makefile.am

@@ -95,6 +95,7 @@ MONGOC_SOURCES_SHARED += \
 	src/mongoc/mongoc-array.c \
 	src/mongoc/mongoc-buffer.c \
 	src/mongoc/mongoc-bulk-operation.c \
+	src/mongoc/mongoc-b64.c \
 	src/mongoc/mongoc-client.c \
 	src/mongoc/mongoc-client-pool.c \
 	src/mongoc/mongoc-cluster.c \