Relocate include/ files to src/, to simplify dist packaging

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/Makefile.am

@@ -1,5 +1,5 @@
 bin_PROGRAMS   = light
-light_SOURCES  = main.c light.c ../include/light.h helpers.c ../include/helpers.h
+light_SOURCES  = main.c light.c light.h helpers.c helpers.h
 light_CPPFLAGS = -I../include -D_XOPEN_SOURCE=500
 light_CFLAGS   = -W -Wall -Wextra -pedantic -std=c89