fix: remove redefinition of Serial

[joscha]

When using this library in the Particle IDE, the following error pops up:

sd-card-library/sd2-card-config.h:21:0: warning: "Serial" redefined [enabled by default]
 #  define Serial Serial    /* Use USB CDC port */
 ^
In file included from ./inc/application.h:45:0,
                 from xxx.cpp:2:
../wiring/inc/spark_wiring_usbserial.h:94:0: note: this is the location of the previous definition
 #define Serial _fetch_global_serial()
 ^
In file included from sd-card-library/sd2-card.h:26:0,
                 from sd-card-library/sd-fat-util.h:27,
                 from sd-card-library/sd-fat.h:27,
                 from sd-card-library/sd-card-library.h:18,
                 from wala.cpp:2:
xxx.cpp: In function 'void setup()':
This looks like an error in sd-card-library library. Would you like to create an issue on GitHub to let the author know?