toolchain/gdb: add pthread to CFLAGS/LDFLAGS for zstd

Works around a build issue when building on a host with an older glibc,
where it would fail to detect ELF support in libbfd

Signed-off-by: Felix Fietkau <[email protected]>

toolchain/gdb/Makefile

@@ -22,8 +22,8 @@ HOST_BUILD_PARALLEL:=1
 
 include $(INCLUDE_DIR)/toolchain-build.mk
 
-export ZSTD_CFLAGS=-I$(STAGING_DIR_HOST)/include
-export ZSTD_LIBS=-L$(STAGING_DIR_HOST)/lib -lzstd
+export ZSTD_CFLAGS=-I$(STAGING_DIR_HOST)/include -pthread
+export ZSTD_LIBS=-L$(STAGING_DIR_HOST)/lib -lzstd -lpthread
 
 HOST_CONFIGURE_VARS += \
 	acx_cv_cc_gcc_supports_ada=false \