Minimal changes to build fastboot as a library.

Bug: http://b/77809061
Test: builds
Change-Id: I0c388334d643a0a2abf0c03a29f336dc3af2f83f
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index cf130c9..42f4fbb 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1410,8 +1410,7 @@
     fprintf(stderr, "FAILED (%s)\n", fb_get_error().c_str());
 }
 
-int main(int argc, char **argv)
-{
+int FastBoot::Main(int argc, char* argv[]) {
     bool wants_wipe = false;
     bool wants_reboot = false;
     bool wants_reboot_bootloader = false;