tests: drivers: build_all: dts_fixup.h: Fix misplaced #endif

Commit 555936f7 ("dts/Kconfig: Remove unused HAS_DTS_SPI")
accidentally removed the wrong #endif, meaning the second part of
dts_fixup.h depended on DT_ADXL372_DEV_NAME not being defined.

Discovered through the reference to the undefined HAS_DTS_SPI symbol in
the comment after the #endif at the end of the file. Adding detection of
unused symbols in samples and tests to CI.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>