uclibc: enable bessel functions

target/mips64/uclibc-ng.config

@@ -54,7 +54,7 @@ ARCH_USE_MMU=y
 UCLIBC_HAS_FLOATS=y
 UCLIBC_HAS_FPU=y
 DO_C99_MATH=y
-# DO_XSI_MATH is not set
+DO_XSI_MATH=y
 # UCLIBC_HAS_FENV is not set
 KERNEL_HEADERS=""
 HAVE_DOT_CONFIG=y