describe example: enable building by default.

examples/.gitignore

@@ -11,4 +11,5 @@ remote
 status
 tag
 for-each-ref
+describe
 *.dSYM

examples/Makefile

@@ -5,6 +5,7 @@ CFLAGS = -g -I../include -I../src -Wall -Wextra -Wmissing-prototypes -Wno-missin
 LFLAGS = -L../build -lgit2 -lz
 APPS = general showindex diff rev-list cat-file status log rev-parse init blame tag remote
 APPS += for-each-ref
+APPS += describe
 
 all: $(APPS)