Makefile: Prompt for reconfigure on lavc/hwaccels.h modification

Adding a new AVHWAccel also adds a new CONFIG variable for it
and said config variables are typically used to calculate the
size of stack arrays. In such a context, an undefined CONFIG
variable does not evaluate to zero; instead it leads to
a compilation failure. Therefore treat this file like the other
files containing lists of configurable components and prompt
for reconfiguration if it is modified.

(E.g. a44fba0b5b led to compilation
failures for me.)

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

Makefile

@@ -78,6 +78,7 @@ tools/target_dem_%_fuzzer$(EXESUF): $(FF_DEP_LIBS)
 CONFIGURABLE_COMPONENTS =                                           \
     $(wildcard $(FFLIBS:%=$(SRC_PATH)/lib%/all*.c))                 \
     $(SRC_PATH)/libavcodec/bitstream_filters.c                      \
+    $(SRC_PATH)/libavcodec/hwaccels.h                               \
     $(SRC_PATH)/libavcodec/parsers.c                                \
     $(SRC_PATH)/libavformat/protocols.c                             \