Re-add doxygen/input to INPUT variable

Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/doxygen/mbedtls.doxyfile b/doxygen/mbedtls.doxyfile
index 78c2205..cc2c51e 100644
--- a/doxygen/mbedtls.doxyfile
+++ b/doxygen/mbedtls.doxyfile
@@ -6,7 +6,7 @@
 EXTRACT_PRIVATE        = YES
 EXTRACT_STATIC         = YES
 CASE_SENSE_NAMES       = NO
-INPUT                  = ../include ../tf-psa-crypto/include ../tests/include/alt-dummy
+INPUT                  = ../include input ../tf-psa-crypto/include ../tests/include/alt-dummy
 FILE_PATTERNS          = *.h
 RECURSIVE              = YES
 EXCLUDE_SYMLINKS       = YES