autotools: add missing header

Signed-off-by: Steve Markgraf <steve@steve-m.de>

include/Makefile.am

@@ -1,5 +1,5 @@
 rtlsdr_HEADERS = rtl-sdr.h rtl-sdr_export.h
 
-noinst_HEADERS = reg_field.h rtlsdr_i2c.h tuner_e4k.h tuner_fc0012.h tuner_fc0013.h tuner_fc2580.h
+noinst_HEADERS = reg_field.h rtlsdr_i2c.h tuner_e4k.h tuner_fc0012.h tuner_fc0013.h tuner_fc2580.h tuner_r820t.h
 
 rtlsdrdir = $(includedir)