Revert -export-symbols-regex 'celt_'. That probably wasn't a good idea

And it somehow prevents Opus from building shared libraries

libcelt/Makefile.am

@@ -21,7 +21,7 @@ libcelt@LIBCELT_SUFFIX@_la_SOURCES = bands.c celt.c cwrs.c ecintrin.h entcode.c
 
 libcelt@LIBCELT_SUFFIX@_la_LDFLAGS = \
 	-version-info @CELT_LT_CURRENT@:@CELT_LT_REVISION@:@CELT_LT_AGE@ \
-	-no-undefined -export-symbols-regex 'celt_'
+	-no-undefined
 
 noinst_HEADERS = _kiss_fft_guts.h arch.h bands.h fixed_c5x.h fixed_c6x.h \
 	cwrs.h ecintrin.h entcode.h entdec.h entenc.h fixed_generic.h float_cast.h \