fix build with gcc 10

Define __Cc, __Dc and Res_bit as extern to avoid the following build
failure with gcc 10 (which defaults to -fno-common):

/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: CMakeFiles/mpcdec_shared.dir/requant.o:(.rodata+0x0): multiple definition of `__Dc'; CMakeFiles/mpcdec_shared.dir/mpc_decoder.o:(.rodata+0x430): first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: CMakeFiles/mpcdec_shared.dir/requant.o:(.rodata+0x28): multiple definition of `__Cc'; CMakeFiles/mpcdec_shared.dir/mpc_decoder.o:(.rodata+0x458): first defined here
/nvme/rc-buildroot-test/scripts/instance-0/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/10.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: CMakeFiles/mpcdec_shared.dir/requant.o:(.rodata+0x74): multiple definition of `Res_bit'; CMakeFiles/mpcdec_shared.dir/mpc_decoder.o:(.rodata+0x4a4): first defined here

Fixes:
 - http://autobuild.buildroot.org/results/9971da8c59484a6a26eb79d6e157461a47855dbf

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

diff -Naur musepack_src_r475.orig/libmpcdec/requant.h musepack_src_r475/libmpcdec/requant.h
--- musepack_src_r475.orig/libmpcdec/requant.h	2020-09-10 08:31:50.696105246 +0200
+++ musepack_src_r475/libmpcdec/requant.h	2020-09-10 08:32:52.944106476 +0200
@@ -47,9 +47,9 @@
 
 
 /* C O N S T A N T S */
-const mpc_uint8_t      Res_bit [18];     ///< Bits per sample for chosen quantizer
-const MPC_SAMPLE_FORMAT __Cc    [1 + 18]; ///< Requantization coefficients
-const mpc_int16_t       __Dc    [1 + 18]; ///< Requantization offset
+extern const mpc_uint8_t      Res_bit [18];     ///< Bits per sample for chosen quantizer
+extern const MPC_SAMPLE_FORMAT __Cc    [1 + 18]; ///< Requantization coefficients
+extern const mpc_int16_t       __Dc    [1 + 18]; ///< Requantization offset
 
 #define Cc (__Cc + 1)
 #define Dc (__Dc + 1)